United States v. Danilo Garcia , 752 F.3d 382 ( 2014 )


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  •                              PUBLISHED
    UNITED STATES COURT OF APPEALS
    FOR THE FOURTH CIRCUIT
    No. 13-4136
    UNITED STATES OF AMERICA,
    Plaintiff - Appellee,
    v.
    DANILO GARCIA, a/k/a Donny, a/k/a Darreo,
    Defendant - Appellant.
    Appeal from the United States District Court for the District of
    Maryland, at Baltimore.    Catherine C. Blake, District Judge.
    (1:11-cr-00569-CCB-1)
    Argued:   March 20, 2014                    Decided:   May 15, 2014
    Before GREGORY and KEENAN, Circuit Judges, and DAVIS, Senior
    Circuit Judge.
    Vacated and remanded by published opinion. Senior Judge Davis
    wrote the opinion, in which Judge Gregory and Judge Keenan
    joined.
    ARGUED: Todd Michael Brooks, WHITEFORD TAYLOR & PRESTON LLP,
    Baltimore, Maryland, for Appellant. Ayn Brigoli Ducao, OFFICE
    OF THE UNITED STATES ATTORNEY, Baltimore, Maryland, for
    Appellee. ON BRIEF: Erek L. Barron, WHITEFORD TAYLOR & PRESTON
    LLP, Bethesda, Maryland, for Appellant.    Rod J. Rosenstein,
    United States Attorney, OFFICE OF THE UNITED STATES ATTORNEY,
    Baltimore, Maryland, for Appellee.
    DAVIS, Senior Circuit Judge:
    Appellant Danilo Garcia, one of fourteen defendants named
    in a superseding indictment returned by the grand jury in the
    District of Maryland, appeals his convictions on five counts of
    narcotics trafficking. After a severance, Garcia proceeded to
    trial with two codefendants.
    The Government’s presentation at trial consisted primarily
    of    two   types    of    evidence:     (1)   law    enforcement       testimony
    recounting    observations      (and    describing    a    few   drug    seizures)
    resulting from close surveillance of the physical comings and
    goings of numerous coconspirators and targets, including drug
    couriers;    and     (2)    audio      recordings     of    wiretapped     mobile
    telephone    conversations      concerning     drug    supplies,      deliveries,
    and   payments      therefor.   Over     defendants’       vigorous     objections
    before and during trial, the district court permitted an agent
    of the Federal Bureau of Investigation to testify as an expert
    on coded drug-related conversations. The agent also testified as
    a fact witness regarding aspects of the lengthy investigation.
    The jury convicted Garcia of drug conspiracy and of the
    four substantive counts in which he was named, but it was unable
    to reach a unanimous verdict as to his co-defendants and the
    court declared a mistrial on those charges.
    On appeal, Garcia assigns error to the district court’s
    admission of the decoding expert’s testimony, and the court’s
    2
    denial   of   his   motion       for    judgment       of   acquittal      for    lack   of
    sufficient evidence as to one of the substantive counts on which
    he was convicted.
    Upon our careful review of the record, we hold that the
    district court abused its discretion in its evidentiary rulings
    as viewed in their totality. Specifically, we hold that, on this
    record, safeguards adopted by the district court to avoid the
    substantial risk of prejudice inhering in the jury’s receipt of
    the decoding expert’s testimony were inadequate. Garcia timely
    and repeatedly objected regarding the foundational sufficiency
    and methodological reliability of the agent’s expert testimony,
    and he specifically pointed to the risk of prejudice arising
    from   the    agent’s     dual    capacity       as    both      an   expert     and    fact
    witness. We are persuaded that, under the circumstances of this
    case, neither the district court’s cautionary instructions to
    the    jury   nor   its    sporadic       sustaining        of    some     of    counsels’
    objections      adequately        mitigated           the   risk      of    substantial
    prejudice.     Furthermore,       we     are     unable     to    conclude       that    the
    missteps evident in this record were harmless. We reach this
    conclusion     reluctantly,            because    the       district       court       tried
    mightily to hew to the lines we have drawn in prior cases.
    Accordingly, although we discern no reversible error in the
    court’s denial of the motion for judgment of acquittal, we hold
    that the errors in the decoding expert’s testimony so infected
    3
    the entire trial that we must vacate the judgment and remand the
    case to the district court for further proceedings.
    I.
    A.
    On August 23, 2012, the grand jury returned a ten-count
    superseding      indictment        against         Garcia       and   thirteen      others.
    Garcia    was    named      in   the     following        five    counts:     Count     One,
    conspiracy      to   distribute        more       than    one    kilogram    or    more   of
    heroin, in violation of 21 U.S.C. §§ 841(a)(1), 846, from April
    2009 to November 2011; Count Four, possession with intent to
    distribute heroin on April 19, 2011 in violation of 21 U.S.C.
    § 841(a)(1); Count Five, possession with intent to distribute
    100 grams or more of heroin on July 22, 2011 in violation of 21
    U.S.C.    § 841(a)(1);           Count    Six,       possession       with    intent      to
    distribute 100 grams or more of heroin on August 5, 2011 in
    violation of 21 U.S.C. § 841(a)(1); and Count Seven, possession
    with intent to distribute 100 grams or more of heroin on April
    17, 2009 in violation of 21 U.S.C. § 841(a)(1).
    B.
    At the times relevant to this case, Garcia lived in New
    York     City.       The     basic       theory          of     the   prosecution         was
    straightforward:           (1)   coconspirator           Yoni    Rodriguez,       who   pled
    guilty in the Eastern District of New York on narcotics charges
    and testified against Garcia pursuant to a plea agreement, was
    4
    Garcia’s      source      of   uncut    heroin   starting      sometime    in   2005,
    selling to Garcia every few weeks or so, usually on consignment;
    (2)    Garcia     would     distribute     the   heroin   in    Baltimore,      after
    either transporting the drugs himself or via drug couriers from
    New York; (3) Roy Lee Clay and Walter Lee Powell were Baltimore-
    based middlemen who distributed quantities of heroin to other
    dealers in the Baltimore area.
    In addition to the charge of knowing participation in the
    thirty-month       drug     trafficking     conspiracy,     Garcia      was   charged
    with       four   specific     instances    of   possession      with     intent   to
    distribute        heroin,      the     circumstances   surrounding        which    we
    summarize as follows:
    On April 17, 2009, a police officer observed
    Garcia exiting a white shuttle bus in the parking lot
    of a Baltimore travel plaza, carrying a brown paper
    bag. After Garcia saw the police officer, he reentered
    the bus, left the bag behind, and disclaimed any
    knowledge of or interest in the bag. The officer found
    a manicure set inside the bag, and inside the manicure
    set he discovered approximately 200 grams of heroin.
    On April 19, 2011, police observed Powell enter
    Kenya Salik Montgomery’s car and exiting after about
    two minutes. When, about two hours later, the police
    searched Montgomery, they found multiple baggies of
    heroin. 1
    1
    Garcia’s convictions on this and the two subsequent
    incidents are based on an aiding and abetting theory, tracing
    the drugs seized back to the heroin Garcia had obtained from
    Rodriguez and then distributed to dealers in Baltimore.
    5
    On July 22, 2011, Garcia spoke by phone with
    Powell    and   instructed    Powell  to     travel   to
    Philadelphia. The FBI observed Powell when he arrived
    in Philadelphia and visited a certain neighborhood.
    After   remaining  in   Philadelphia  a   mere   fifteen
    minutes, Powell returned to Maryland (followed by law
    enforcement).    A  Maryland    State   Police   trooper
    conducted a traffic stop upon Powell’s entry into the
    state and the trooper recovered 143.7 grams of heroin
    from Powell.
    On August 5, 2011, FBI agents observed Nancy
    Feliciano at a travel plaza in Baltimore, leaving a
    van and getting into a cab. As she was getting into
    the cab, the agents approached her and searched her
    bags (with her consent). They found approximately 500
    grams of heroin and an index card which had Powell’s
    address and phone number. Later, while she was being
    detained in the custody of the FBI, Feliciano made a
    monitored call to Garcia and accused him of providing
    her with heroin.
    During   the   trial,   many        members   of   law   enforcement,
    including FBI agents, U.S. Drug Enforcement Agency agents, and
    state and local law enforcement officers, testified regarding
    their surveillance and observations of the defendants together
    in and around Baltimore, as well as in New York and once in
    Philadelphia. Critically, much of the investigation entailed the
    maintenance of wiretaps on four mobile phones used by Garcia,
    two used by Clay, and two used by Powell. To a significant
    extent, coded telephone conversations could be aligned with the
    physical activities of the coconspirators.
    C.
    On August 30, 2012, the Government provided notice to the
    defendants of its intent to call FBI Special Agent Carrie Dayton
    6
    as an expert. The Government provided Agent Dayton’s resumé, and
    stated that she “served as a monitor on the wiretaps, and will
    testify as to the meaning of coded references in several of the
    calls used by the conspirators, when discussing drug trafficking
    over the phone.” J.A. 38. The Government’s disclosure showed
    that Agent Dayton had worked as a paralegal and then, for more
    than six years, as an attorney, before joining the FBI in May
    2004. She thus had eight years of law enforcement experience by
    the   time    of     trial.      She    initially      worked    in   the   FBI’s   Civil
    Rights and White Collar Crime divisions, and joined the Safe
    Streets      Task    Force       in    November    2007,    working     narcotics    and
    violent crime investigations.
    Agent Dayton had “observed and [] recognize[d] patterns of
    behavior consistent with narcotics trafficking” as a result of
    her   participation         in    over    20     investigations.      J.A.    57.   Agent
    Dayton was also involved in over five wiretap investigations,
    during which she “listened to thousands of phone conversations
    between individuals who participated in or were suspected of”
    drug trafficking conspiracies and other crimes. 
    Id. Agent Dayton
    attested      that    she     has      “gained     a   working    knowledge    of    drug
    language and terminology commonly used by drug dealers” and that
    she was “aware of the technique and [has] heard hundreds of
    conversations in which narcotics traffickers employ code words
    7
    to disguise discussions” about drugs. 
    Id. Agent Dayton
    had not
    previously testified as an expert on coded drug language.
    Days before the commencement of trial, defendants moved to
    exclude the testimony of Agent Dayton on the basis that the
    Government failed to provide a sufficient expert disclosure in
    conformity with Federal Rule of Criminal Procedure 16(a)(1)(G).
    On October 2, 2012, after opening statements, the district court
    denied   the     motion.   Referring     to    whether     the   Government    had
    provided     a    sufficient    outline       of   Agent    Dayton’s     proposed
    testimony, the court stated that,
    it appears to me that when a disclosure says she is
    going to testify to coded references in transcripts of
    calls related to narcotics, clearly, based on the
    openings, counsel have understood the government’s
    theory to be that references to things like cars and
    perhaps girls are in fact references to drugs, that it
    does not take any great additional leap of inference
    from there to figure out what Special Agent Dayton is
    going to testify to.
    J.A. 177-78. Accordingly, the district court held that there was
    no   prejudice      to   the   defense    “even    if    there   was    something
    deficient in the disclosure.” J.A. 178.
    D.
    On eighteen separate occasions during six days over the two
    week trial, the Government called and re-called Agent Dayton to
    the stand to testify on the coded language used in recorded
    wiretapped       calls   between   the    conspirators.      From      the   start,
    defense counsel voiced vigorous and repeated objections to Agent
    8
    Dayton’s qualifications as an expert witness in decoding drug
    traffickers’           language.        Counsels’       objections           continued    as    the
    presentation of her testimony unfolded, specifically based on
    Agent Dayton’s failure to conform to recognized methodologies
    for such testimony and the Government’s failure to explicitly
    demonstrate the existence of an acceptable foundation for many
    of her specific interpretations. 2 The district court generally
    overruled          the     defense       objections         (while       sustaining       some),
    holding that Agent Dayton had “sufficient experience to meet the
    standard to be able to offer opinion testimony that might be
    helpful to the jury based on . . . her five years of experience,
    dozens      and     dozens    of     shifts      on     wiretaps,        and    thousands       and
    thousands         of     calls,    as    well    as     her      conversations       with      both
    cooperators,           witnesses,       and     other    law      enforcement        officers.”
    J.A. 320.
    Manifesting a deep familiarity with this Circuit’s settled
    guideposts          with     regard       to     this       type        of     testimony,       the
    experienced         judge    identified         early       on    two    potential       problems
    with       Agent    Dayton’s       testimony:         (1)     the   need       to   distinguish
    between her lay fact testimony based on her personal knowledge,
    on the one hand, and her expert opinion testimony based on her
    2
    It appears, and we presume, that there was an
    understanding at trial that an objection made by any one of the
    three defense counsel was joined by the others.
    9
    training and investigatory experience, on the other; and (2)
    ensuring that she was testifying on the basis of her experience
    and expertise in coded language, and not simply repeating what
    cooperators or witnesses told her.
    This second point was especially salient because, as Agent
    Dayton conceded during voir dire in the presence of the jury,
    she   had       debriefed    several      cooperating       coconspirators         in    this
    very case, including at least one of whom speaks only Spanish,
    Garcia’s        supplier,    Yoni   Rodriguez.           Thus,   it     was   particularly
    apparent at the start of trial (especially as Agent Dayton had
    never     before     testified      as    an    expert)     that      special     care   was
    required to expose to the jury (and to the defense) the true
    bases for Agent Dayton’s “opinions.”
    In any event, cognizant of these perils, the district court
    issued      a    cautionary    instruction          to    the    jury    prior    to    Agent
    Dayton’s        testimony.    It    informed        the   jury    that    a   distinction
    would     be     made   between     the    agent’s        fact    and    expert    opinion
    testimony, and that it would not be appropriate for Agent Dayton
    to testify to the meaning of a word simply because a witness
    told her its meaning. 3
    3
    The district court told the jury:
    The fact that I am determining that she has sufficient
    qualifications to offer you opinion testimony, again,
    I’m not deciding what weight you give her testimony or
    (Continued)
    10
    how you evaluate it. It’s up to you, as I said at the
    beginning, to pay attention to her experience, and her
    background, and training, and the reasons that she
    gives you for any opinion that she may offer, and then
    you decide what weight to give it.
    The other thing I want to say is that she may also be
    offering more fact testimony, just an ordinary lay
    witness or fact testimony about certain matters, and I
    have asked counsel to be clear in their questions
    which is which.
    To the extent she is offering you just fact testimony,
    something she actually saw or heard, that should be
    clear. To the extent it’s her opinion, based on her
    experience and review of these calls, we’ll make that
    clear as well, and she will be asked to explain what
    the reason for her opinion is.
    ***
    But I want to be clear, any opinion that she may be
    offering you about these calls is not to be based on
    what somebody, who may or may not have been involved
    in this case, told her. Mr. Jones said that means
    cocaine, for example.
    That’s not the basis of her opinion. That’s not
    appropriate. She is not here to just repeat something
    that somebody may or may not have said to her when we
    don’t have that person here for you to listen to, and
    make your own opinion about credibility.
    So her opinion, and we will be careful to limit it to
    that, is to the extent she has got a basis, on her
    experience and training and everything she told you
    about yesterday, to have a reason to believe that a
    certain reference means something that is not apparent
    on its face, she will explain that, and it is up to
    you to evaluate it again. But it is not because it is
    based on anybody that she may or may not have talked
    to in this case about the meaning of certain words.
    J.A. 326-28.
    11
    In   the    course   of     the    investigation    in     this   case,
    investigators had monitored in whole or in part more than 20,000
    calls; the Government considered 1,928 conversations pertinent
    to the investigation and it played approximately 211 of such
    monitored calls, many in Spanish, during the trial. Agent Dayton
    testified repeatedly over the course of six days, “interpreting”
    words in nearly half the calls played before the jury, including
    the   English    translations     of    the   conversations    occurring   in
    Spanish.   The   defense   made   countless     objections     regarding   the
    melding of her fact and expert opinion testimony, the agent’s
    failure to adhere to her professed methodology, and the absence
    of foundations for many of her specific interpretations. While
    the district court sustained some objections, 4 for the most part,
    4
    One notable objection early in Agent Dayton’s testimony
    came after she testified as follows after listening to a wiretap
    recording:
    AGENT DAYTON: In the second line of Mr. Coley
    speaking, wherein he says I owe you 200, he is
    actually talking about $2,000. In that same sentence,
    when he says I have a hundred and 50 for you, he is
    actually referring to $1500.
    GOVERNMENT: Are these amounts in reference to, based
    on your expert opinion, in reference to, these dollar
    amounts, in reference to anything in particular?
    AGENT DAYTON: These are in reference to – Mr. Powell
    has supplied Mr. Coley with heroin, and Mr. –
    J.A. 364. Defense counsel objected at this point, arguing that
    Agent Dayton had gone “beyond the spectrum of what she is
    (Continued)
    12
    capable of doing” by “opin[ing] as to . . . the general nature
    of this particular conversation, and what the conversation
    means[.]” J.A. 365. Counsel further argued that “there has been
    no foundation or no explanation as to how she came to some of
    these conclusions. She is . . . just saying this is what this is
    about, and we are listening to these calls isolated, without any
    proper context.” 
    Id. The district
    court astutely summed up the issue:
    The problem is that [Agent Dayton] just said, in
    response to your question about how she knows that the
    200 is a reference to 2,000, she just said because I
    know that Mr. Coley supplied heroin to Mr. Powell.
    That is not based on her expertise or the coded
    language. She is using fact information that she has
    gained otherwise, which is not appropriate.
    J.A. 366-67. The prosecutor responded by stating that she was
    trying to stay away from the objections raised by
    counsel regarding her, Special Agent Dayton testifying
    about what she learned from persons in this case, but
    it does form the basis, part of the basis of her
    opinion. I can certainly elicit that, but then I would
    think there’s going to be an objection to her
    testifying about that being the basis of her opinion.
    J.A. 367-68.
    The district court ultimately sustained the objection (as
    it did some others along the same vein) after the prosecutor
    further stated that “[r]egarding the numbers that Mr. Coley
    uses, she is in part relying on the investigation which showed
    that Mr. Coley and Mr. Powell met,” to which the district court
    responded, “Then it’s hearsay and it’s not an expert opinion.”
    J.A. 369.
    The fact that the prosecutor hesitated to lay an adequate
    foundation for Agent Dayton’s opinions for fear of objection
    from defense counsel speaks volumes to the impermissible basis
    for that foundation. See also J.A. 375 (district court sustained
    objection because Dayton’s testimony that “cousin” meant a
    specific individual was on the basis of her personal knowledge,
    (Continued)
    13
    it   admitted      the    bulk     of   Agent       Dayton’s    testimony,         which   was
    often     elicited       through    a   prosecutor’s           insertion     of     a   simple
    prologue     to      a     question:       “Agent       Dayton,       in     your       expert
    opinion . . . ”            or       “Agent           Dayton,         based         on      your
    expertise . . . .” 5
    As    mentioned,       Agent      Dayton’s       testimony       was    interspersed
    with the testimony of other investigators. After a 12-day trial,
    the jury returned a verdict of guilty on all five counts in
    which Garcia was named (one count of conspiracy to distribute
    heroin, and four counts of possession with intent to distribute
    heroin),     but     a    mistrial       was        declared    as    to     the    two    co-
    defendants. The district court entered judgment and sentenced
    Garcia to 188 months on all counts, to run concurrently.
    Garcia       timely       appealed        and     we     exercise       jurisdiction
    pursuant to 28 U.S.C. § 1291.
    not on expertise); J.A. 452 (district court overruled objection
    based on lack of foundation despite term only heard in this
    investigation); J.A. 461 (district court overruled objection and
    advised Agent Dayton to “rely on what [she] learned prior to
    this   investigation”);  J.A.   573  (district   court overruled
    objection where foundation for interpretation was that “Mr.
    Garcia uses the term the [sic] G. We see him use that in other
    calls on this wire to refer to a thousand dollars.”).
    5
    As we discuss infra n.10, this formulaic approach hardly
    cured the problems that the district court anticipated and that
    we find actually to have arisen in the course of trial.
    14
    II.
    Garcia presents a number of arguments on appeal regarding
    the admissibility of Agent Dayton’s expert testimony. While we
    find that the district court did not abuse its discretion in
    qualifying Agent Dayton as an expert, we are persuaded that the
    district court failed to adequately safeguard against a strong
    likelihood of jury confusion between Agent Dayton’s testimony as
    an expert witness and as a fact witness, and in ensuring that,
    in her capacity as an expert witness, Agent Dayton “reliably
    applied” her methodology and adequately set forth the underlying
    permissible foundations for her opinions. 6
    6
    Garcia also challenges the sufficiency of the evidence on
    Count 4 (the April 19, 2011 Powell/Montgomery transaction), for
    possession of heroin with intent to distribute. Garcia’s
    conviction on that count, as on two of the other three of the
    substantive counts, was based on an aiding and abetting theory.
    We find the evidence was sufficient to prove beyond a reasonable
    doubt that Garcia “knowingly associated himself with and
    participated in the criminal venture.” United States v.
    Winstead, 
    708 F.2d 925
    , 927 (citing Nye & Nissen v. United
    States, 
    336 U.S. 613
    , 619 (1949) (further citations omitted));
    see also Rosemond v. United States, 
    134 S. Ct. 1240
    , 1245-51
    (2014)    (explaining    aiding     and    abetting     principles).
    “[P]articipation in every stage of an illegal venture is not
    required, only participation at some stage accompanied by
    knowledge of the result and intent to bring about that result.”
    United States v. Burgos, 
    94 F.3d 849
    , 873 (4th Cir. 1996) (en
    banc) (internal citations omitted). Rodriguez testified that he
    sold heroin to Garcia, which Garcia then sold to Powell for
    further   distribution   to   customers    like   Montgomery.    The
    relationship between Garcia and Powell was established by the
    wiretap calls, as well as surveillance of Garcia and Powell
    together.   Furthermore,   on   the   very   day    Montgomery   was
    apprehended in possession of heroin, a conversation between
    (Continued)
    15
    A.
    Garcia   contends   that   the   district   court   abused   its
    discretion in qualifying Agent Dayton as a decoding expert in
    the first place, and that therefore the district court erred in
    admitting her testimony, without regard to the special dangers
    inherent in the use of decoding experts. 7 We disagree.
    Garcia and Powell was recorded on Montgomery’s voicemail, in
    which Garcia and Powell discussed their drug trafficking plans
    as interpreted by Agent Dayton.
    We note that even though we find error in the manner in
    which Agent Dayton’s testimony was presented to the jury, we
    must still consider that testimony in reviewing the denial of a
    Rule 29 motion for judgment of acquittal. United States v. Diaz,
    
    300 F.3d 66
    , 77 (1st Cir. 2002); see also United States v.
    Horton, 
    693 F.3d 463
    , 482 (4th Cir. 2012) (Davis, J.,
    concurring) (discussing the necessity of ruling on appellate
    claims of evidentiary insufficiency even in the context of trial
    error requiring reversal).
    Of course, we intimate no view as to whether upon a retrial
    the Government will adduce sufficient evidence to support
    conviction as to any count of the indictment.
    7
    Garcia also argues that the government’s pretrial
    disclosures regarding Agent Dayton were insufficient. Federal
    Rule of Criminal Procedure 16(a)(1)(G) requires that “[a]t the
    defendant’s request, the government must give to the defendant a
    written summary of any testimony that the government intends to
    use under Rules 702, 703, or 705 of the Federal Rules of
    Evidence during its case-in-chief at trial. . . . The summary
    provided under this subparagraph must describe the witness’s
    opinions, the bases and reasons for those opinions, and the
    witness's qualifications.” Garcia maintains that the information
    contained in the Government’s September 24, 2012 letter, and
    Dayton’s attached resumé, was insufficient to meet this
    requirement.
    (Continued)
    16
    We review a district court’s decision to qualify an expert
    witness, as well as the admission of such testimony, for abuse
    of discretion. United States v. Wilson, 
    484 F.3d 267
    , 273 (4th
    Cir. 2007). “A court abuses its discretion if its decision is
    ‘guided by erroneous legal principles’ or ‘rests upon a clearly
    erroneous factual finding.’” United States v. McLean, 
    715 F.3d 129
    , 142 (4th Cir. 2013) (quoting United States v. Johnson, 
    617 F.3d 286
    , 292 (4th Cir. 2010)).
    The Federal Rules of Evidence provide that a
    witness who is qualified as an expert by knowledge,
    skill, experience, training, or education may testify
    in the form of an opinion or otherwise if:
    (a) the expert’s scientific, technical, or other
    specialized knowledge will help the trier of fact to
    understand the evidence or to determine a fact in
    issue;
    (b) the   testimony   is   based   on    sufficient    facts   or
    data;
    (c)   the  testimony   is   the         product   of   reliable
    principles and methods; and
    (d) the expert has reliably applied the principles and
    methods to the facts of the case.
    In light of our ultimate ruling on the propriety of Agent
    Dayton’s testimony, we need not address the adequacy of the
    Government’s compliance with Rule 16(a)(1)(G).
    17
    Fed. R. Evid. 702. In the advisory committee notes following the
    2000 amendments to the Rule, there is a pointed discussion on
    the use of hybrid witnesses at issue here:
    For example, when a law enforcement agent testifies
    regarding the use of code words in a drug transaction,
    the principle used by the agent is that participants
    in such transactions regularly use code words to
    conceal the nature of their activities. The method
    used by the agent is the application of extensive
    experience    to   analyze    the   meaning    of  the
    conversations. So long as the principles and methods
    are reliable and applied reliably to the facts of the
    case, this type of testimony should be admitted.
    Fed. R. Evid. 702 advisory committee’s note (emphasis added).
    Garcia’s    argument      that    Agent       Dayton’s       experience    was
    insufficient    to   qualify   her    as    a    decoding    expert,    given   the
    emphasis on experiential knowledge for these types of experts,
    misses the mark. We think Fourth Circuit law in this area is
    reasonably   clear.    
    Wilson, 484 F.3d at 267
    ;   see   also   United
    States v. Galloway, --- F.3d ---, ---, No. 12-4545, 
    2014 WL 1424939
    , at *4-6 (4th Cir. April 15, 2014) (applying plain error
    review).
    In Wilson, we reviewed the district court’s admission of a
    police   investigator’s     expert    and       fact    witness   testimony;    the
    investigator’s expertise was in “decipher[ing] the intercepted
    communications that were recorded via wiretap” as they “were
    full of drug slang and street code[.]” 
    Id. at 273.
    We found no
    abuse of discretion in the district court’s admission of the
    18
    expert    evidence.      There,     the    expert’s    qualifications      included
    nine   years     spent     mostly     investigating      drug   traffickers     and
    attendance in training programs offered by the DEA, training
    which specifically included exposure to drug traffickers’ use of
    coded language. 
    Id. at 275-76.
    While Agent Dayton certainly has less experience than the
    witness in Wilson, and had never before this case been qualified
    as an expert, the precise number of years of an investigator’s
    experience, or the number of investigations on which she has
    worked, is not necessarily dispositive. Every expert has a first
    time. It is the quality of Agent Dayton’s experience, especially
    her exclusive focus on narcotics trafficking for the preceding
    five   years,    on     which   the   district    court   properly    focused     in
    finding her qualified as a decoding expert.
    Agent Dayton’s job required her “to work in close proximity
    with drug users on a daily basis,” United States v. Baptiste,
    
    596 F.3d 214
    , 218 (4th Cir. 2010); she had “monitored dozens of
    wiretaps, listening to thousands of intercepted phone calls in
    the process,” United States v. Mack, 495 Fed. App’x 359, 364
    (4th   Cir.     2012)     (internal       quotations   omitted);     and   in   the
    “course    of    [her]      extensive       experience     dealing    with      drug
    traffickers and confidential informants, [] was able to learn to
    understand the vernacular used by drug traffickers.” 
    Wilson, 484 F.3d at 275-76
    .
    19
    Garcia also takes issue with Agent Dayton’s explanation of
    her methodology. The defendants in Wilson similarly challenged
    the investigator’s methodology, claiming that it was “neither
    sufficiently explained nor 
    reliable.” 484 F.3d at 273
    . Agent
    Dayton explained her methodology as follows: “I think we see
    some common [terms], receipts or paper or clothing terminology.
    So[me] of it’s common and some is just in the context of the
    conversation. As you listen to the conversation, the language
    used doesn’t make sense, and sometimes they switch up code so
    the context is all wrong.” J.A. 285. This is very similar to the
    witness’s explanation in 
    Wilson. 484 F.3d at 275
    (explaining
    that the expert relied on “the context of the call” to see the
    “pattern   that   develops.”).      While     this    explanation    is   not   as
    coherent as we might wish, given the deferential standard of
    review applicable here, it passes muster as an articulation of
    an accepted methodology. Agent Dayton also based her opinions in
    part on conversations she had with witnesses in other cases who
    “would talk about the drug shops and the language that was used
    to communicate with one another.” J.A. 294.
    Given the similarity between Agent Dayton’s explanation of
    her   methodology   and    the    methodology        we   found   acceptable    in
    Wilson,    we   cannot    say    that   the   district      court   abused      its
    discretion in qualifying Agent Dayton as an expert. As we have
    recognized, “[a] ‘trial judge must have considerable leeway in
    20
    deciding     in    a    particular    case    how   to    go    about    determining
    whether particular expert testimony is reliable.’” 
    Wilson, 484 F.3d at 273
    (quoting Kumho Tire Co. v. Carmichael, 
    526 U.S. 137
    ,
    152 (1999)). Agent Dayton met the requirements for testifying to
    the meaning of coded language recorded from the coconspirators’
    calls, and adequately explained her methodology. Accordingly, no
    abuse of discretion infects the district court’s determination
    to accept Agent Dayton as a decoding expert.
    B.
    Despite      the    district    court’s    careful       attention    to   Agent
    Dayton’s credentials as a decoding expert, however, we hold that
    the agent’s testimony was fraught with error arising from the
    problems     the    district    court    itself      identified     early    in    the
    trial:     the     conflation    of     Agent    Dayton’s       expert     and    fact
    testimony, particularly her reliance on her knowledge of the
    investigation to support her coding interpretations; her failure
    to   apply   her       methodology   reliably;      and   last,    her   failure    to
    state on the record an adequate foundation for very many of her
    specific     interpretations.         Moreover,      because       Agent    Dayton’s
    testimony was so extensive and most likely highly influential in
    the jury’s evaluation of the Government’s case against Garcia,
    we are constrained to hold that these flaws deprived Garcia of a
    fair trial, i.e., that the missteps were not harmless, and thus
    require vacatur of Garcia’s convictions.
    21
    1.
    Garcia contends that there were inadequate safeguards to
    protect the jury from conflating Dayton’s testimony as an expert
    and fact witness. We agree.
    We have recognized that individuals who testify as expert
    and fact witnesses can cause jury confusion, and such a manner
    of proceeding is only “acceptable where ‘the district court took
    adequate steps . . . to make certain that [the witness’s] dual
    role did not prejudice or confuse the jury.’” 
    Baptiste, 596 F.3d at 224
    (quoting 
    Wilson, 484 F.3d at 278
    n.5) (alternations in
    original). Such safeguards might include requiring the witness
    to   testify    at    different     times,       in    each   capacity;      giving   a
    cautionary instruction to the jury regarding the basis of the
    testimony;     allowing    for    cross-examination           by   defense   counsel;
    establishing a proper foundation for the expertise; or having
    counsel ground the question in either fact or expertise while
    asking the question. 
    Id. at 224
    (citing 
    Wilson, 484 F.3d at 278
    n.5 and United States v. Farmer, 
    543 F.3d 363
    , 370-71 (7th Cir.
    2008)).
    While a cautionary instruction might be sufficient in many
    instances      to    protect     against        jury   confusion     and     resulting
    prejudice, see Galloway, --- F.3d at ---, 
    2014 WL 1424939
    at *5-
    22
    6, 8   we     are    persuaded      that      the    instruction       given     here   was
    insufficient to mitigate the potential for prejudice.
    The district court represented to the jury that Government
    counsel would “be clear in their questions” whether they were
    asking Agent Dayton to testify based on the facts versus her
    expertise.          J.A.   327.    Despite      this    direction,      the    Government
    failed to do so: there were repeated instances of Agent Dayton
    moving back and forth between expert and fact testimony, with no
    distinction in the Government’s questioning or in Agent Dayton’s
    answers. And this was true despite the fact that the Government
    recalled Agent Dayton to the stand eighteen times. In light of
    the     court’s       earlier      assertion         that    counsel     would    clearly
    distinguish         the    two    types    of    testimony,     the    jury    reasonably
    might       have    assumed      that   all     of   Agent    Dayton’s    testimony     in
    response to questions asking for her expert opinion was indeed
    based on her decoding expertise.
    8
    In Galloway, the district court repeatedly instructed
    counsel to be clearer in the demarcation between fact and lay
    opinion testimony. See, e.g., Joint Appendix at 472-75, 572-73,
    
    2014 WL 1424939
    , ECF No. 59. There, the district court generally
    did not allow the expert to testify about the meaning of
    individual words in particular conversations, but rather only
    permitted the expert to testify whether, in his expert opinion,
    coded language was used in the conversation at issue as a
    general matter. There was little opportunity, then, for the
    expert’s participation in the underlying investigation to
    encroach upon, enhance, or improperly bolster the expert
    testimony of that witness.
    23
    Our     review       of       the     record       reveals         multiple         occasions    in
    which the Government prompted Agent Dayton to assert information
    garnered      from        her    participation             in    the      investigation,            having
    nothing       to     do     with       her     ostensible            decoding         expertise.       For
    instance, in response to a question about any coded language on
    the    call        “based       on     [her]     expertise,”              Dayton       answered      that
    “[w]hen Mr. Powell uses the term show time, he is letting Mr.
    Coley know he has heroin.” J.A. 371-72. Immediately after this
    exchange,       without          any       further     explanation             of    the     term   “show
    time”    or        warning       that        they     were      shifting            away    from    Agent
    Dayton’s expertise to her factual knowledge, the prosecutor then
    asked Agent Dayton: “Now how were Ms. Montgomery and Mr. Coley
    identified as the participants in the calls that we’ve just seen
    or heard?” J.A. 372. Agent Dayton’s response had nothing to do
    with    her     expertise,             and    everything             to   do    with       her     factual
    knowledge as an investigator in this case.
    It is apparent even from the Government’s briefing that
    Agent Dayton used her personal knowledge of the investigation to
    form    (not       simply       to     “confirm”)         her    “expert”            interpretations:
    “Special      Agent       Dayton        also    looked          to    the      actual       seizures   of
    heroin in this case to form the basis of her expert opinion.”
    Appellee Br. 46 n.8.
    Notably, the Government points to a call in which Agent
    Dayton interpreted “a hundred forty five point,” to mean “145
    24
    grams of heroin,” J.A. 885, which is confirmed with evidence
    that       police    seized        145    grams     of    heroin      from    one    of       the
    coconspirators on the call. But unlike the display of expertise
    we   endorsed       in     United    States       v.    Johnson,     this    is    not    Agent
    Dayton “applying [her] expertise, derived over many years and
    from multiple sources, to interpret the transcripts of phone
    conversations.” 
    587 F.3d 625
    , 636 (4th Cir. 2009). Instead, this
    exemplifies occasions, and we discern many of them, in which
    Agent      Dayton     simply       substituted         information    gleaned       from      her
    participation         in    the     investigation         (including        post-indictment
    debriefings of participants in the conspiracy) for ostensible
    expertise. 9        For    this,    the    Government       need     not    have    called      a
    “decoding” expert at all: simply pointing to the seizure of 145
    grams of heroin, and then the repeated mention of “145” in this
    call clearly would have been enough for any juror to make the
    connection. But cloaking this connection in the guise of expert
    testimony goes beyond what is contemplated under Federal Rule of
    Evidence 702, which requires an expert to “reliably appl[y] the
    principles      and       methods”       for   which      she   was    qualified         as    an
    9
    But even if she did not, the record is bereft of evidence
    that she did not, due largely to the Government’s deliberate
    failure to lay an adequate foundation for many of her opinions.
    For our purposes, this absence of clarity as to the source of
    her opinions amounts to the same thing as if she had no
    expertise at all.
    25
    expert. The Rule contemplates that an expert’s opinion testimony
    will    be   “helpful   to   the   jury,”   not     merely   helpful    to   the
    prosecutor     as   transmutations    of    simple    fact   testimony.      Cf.
    
    Wilson, 484 F.3d at 276-78
    and nn.4 and 5 (emphasizing need for
    the district court to enforce methodological, foundational, and
    helpfulness standards in admitting testimony of decoding expert,
    and    affirming    convictions    under    plain    error   review    because,
    despite district court’s erroneous admission of some testimony,
    expert’s “methodology was reliably applied in the vast majority
    of instances”) (emphasis added).
    This issue actually surfaced early in the trial. During the
    Government’s voir dire of Agent Dayton in its effort to satisfy
    the district court that the agent had genuine “expertise,” the
    following colloquy occurred:
    GOVERNMENT:   Special  Agent   Dayton, regarding the
    wiretaps in this case, you have spoken to at least
    three members of the organization?
    DEFENSE: Objection, Your Honor, leading.
    THE COURT: Overruled.
    GOVERNMENT: Is that correct?
    AGENT DAYTON:    At least three, maybe more.
    GOVERNMENT: Maybe more. To confirm your understanding
    of the coded language used in this case?
    AGENT DAYTON: Yes.
    26
    J.A. 296. Later, the defense had Agent Dayton identify the three
    coconspirators       whom    she    had   debriefed,     and     she   stated   the
    following:       “Diego Amparo, Yoni Rodriguez, Nancy Feliciano. I’m
    trying to think of the other folks who have come in. I’m sorry.
    Those are the three I believe we discussed code with.” J.A. 298.
    In   light      of   this   significant        ingredient   in    Agent   Dayton’s
    expertise, it was incumbent upon the Government to demonstrate
    that    Agent    Dayton     was   not   merely    channeling     information    and
    statements by non-testifying participants in the conspiracy into
    the trial record. 10
    10
    In what is truly a curious argument, the Government
    contends that because Agent Dayton “did not offer any expert
    opinion regarding any calls by Yoni Rodriguez, and discussed
    only a handful of calls involving Diego Amparo and Nancy
    Feliciano,” Appellee Br. 46, there should be no concern about
    Agent Dayton’s ostensible methodologically sound expertise.
    Exactly   the  opposite  is   true.  There  is   no  discernible
    correlation favorable to the Government between Agent Dayton’s
    debriefing of coconspirators whose calls she interpreted and
    those whose calls she did not interpret. The point is that it
    was one conspiracy and there is no reason to believe that
    Rodriguez, in particular, would have been unable to provide
    substantial information concerning the operational details of
    the conspiracy. And in fact, he did so in his testimony,
    including “interpretations” of many of the same words Agent
    Dayton interpreted. In short, the funneling of fact information
    learned from those inside the conspiracy and the offer of expert
    testimony was melded in a highly prejudicial manner in this
    case.
    Relatedly, the Government seems earnestly to contend that
    simply by including in its questions to Agent Dayton that the
    agent answer only based on her “expert opinion” somehow
    insulates the agent’s testimony from ordinary scrutiny under the
    settled principles of Daubert v. Merrell Dow Pharm., Inc., 509
    (Continued)
    27
    In Johnson, we expressed wariness over this exact problem,
    and cautioned against “[a]llowing a witness simply to parrot
    out-of-court testimonial statements of cooperating witnesses and
    confidential informants directly to the jury in the guise of
    expert   opinion”     as   it   “would   provide   an   end   run   around
    
    Crawford.” 587 F.3d at 635
    (internal quotation marks omitted).
    In   Crawford   v.   Washington,   the   Supreme   Court   held   that   the
    Confrontation Clause permits the introduction of “[t]estimonial
    statements of witnesses absent from trial . . . only where the
    declarant is unavailable, and only where the defendant has had a
    prior opportunity to cross-examine.” 
    541 U.S. 36
    , 59 (2004). Of
    the three co-conspirators Agent Dayton specifically identified
    as contributing to her “understanding of the coded language used
    in this case,” though by her own admission there may have been
    more than three, only one testified at trial. We explained that
    in the face of such risks, “[t]he question is whether the expert
    is, in essence, giving an independent judgment or merely acting
    U.S. 579 (1993), and its progeny and Rule 702. The Government is
    wrong. It is bootstrapping of the worst kind to suggest to a
    jury that it should believe that everything a witness says is
    based on expertise gained from independent knowledge and
    experience in the absence of a record demonstrating as much. We
    would never permit counsel calling a physician or an engineer or
    an expert tightrope artist to proceed in such a manner; there is
    scant  reason   we   should  allow   a  prosecutor  calling  law
    enforcement officers as experts to do so.
    28
    as a transmitter for testimonial hearsay.” 
    Johnson, 587 F.3d at 635
    .
    Here,    we   cannot       say   that       Agent     Dayton      was    giving     such
    independent     judgments.         While      it   is   true      that    she    never     made
    “direct reference to the content of [her] interviews,” 
    id., this could
    just as well have been the result of the Government’s
    failure    to     elicit      a     proper     foundation         for     Agent    Dayton’s
    interpretations. See J.A. 
    367-68; supra
    12-14 n.4 (discussing
    the prosecutor’s statement in response to an objection for lack
    of    foundation,     that        she   was    “trying       to   stay     away    from    the
    objections raised by counsel regarding her, Special Agent Dayton
    testifying about what she learned from persons in this case, but
    it does form the basis, part of the basis of her opinion. I can
    certainly elicit that, but then I would think there’s going to
    be an objection to her testifying about that being the basis of
    her opinion.”).
    Although      Agent    Dayton       asserted        that    her     after-the-fact
    debriefing of coconspirators who had commenced cooperating with
    the     prosecution        served        to        “confirm[]”           her     independent
    assessments of the meanings to be attributed to many words, it
    was imperative that the trial record demonstrate this sequence
    with a measure of clarity. The record is devoid of evidence that
    this    was,    in    fact,       the   sequence        of    Dayton’s         analysis,     to
    Garcia’s prejudice.
    29
    2.
    We    find        a    second,        equally      fundamental        flaw     in     the
    presentation         of       Agent   Dayton’s        testimony    in   addition       to     the
    above        infirmity:         the   lack      of      foundations      laid        for     each
    interpretation testified to, so much so that we are compelled to
    conclude       that   the       record    fails       to   demonstrate        the    requisite
    reliability          in       Agent    Dayton’s         execution       of     her     claimed
    methodology. Call Number 214, referenced above regarding Agent
    Dayton’s       interpretation            of     the     term    “show    time”        to     mean
    “heroin,” is an illustrative example of the utter absence of any
    foundation        for         more    than      simply      a     few    of     her        coding
    interpretations. No further explanation regarding the term “show
    time” was given – Agent Dayton did not explain that she had seen
    that term used in this investigation, let alone in her previous
    experience. There was no explanation as to what, in the context
    of the call or otherwise, led her to believe that “show time”
    meant “heroin.” 11
    11
    And even where Agent Dayton stated (as she did from time
    to time) that a term had been seen in other calls during the
    investigation, this assertion alone is no explanation for the
    threshold interpretation. Indeed, the fact that the expert has
    heard a term in other calls during the investigation does not,
    by itself, explain how the expert knows what the term means. As
    discussed in text regarding the “interpretation” of numerals,
    the same term means different things at different times, even in
    the same conspiracy. The Government failed to show how Agent
    Dayton reliably leapt these lexicographical crevasses.
    30
    One   significant       indication      of    Agent     Dayton’s     failure     to
    adequately explain her methodology is her lack of explanation as
    to how her methodology was affected by the fact that many of the
    telephone      conversations        were     in      Spanish.        As   Agent    Dayton
    explained, the context of a conversation is an important factor
    when decoding suspected drug language. Although the jury heard
    English language transcripts of the actual call recordings read
    aloud, Agent Dayton provided no insight as to what steps, if
    any, she took to ensure that the context of the conversation was
    not lost or meaningfully altered in the process of translating
    the calls from Spanish to English.
    The     problem    with     this    lack       of    foundation      and    reliably
    applied methodology becomes even more apparent when examining
    the     instances        when     Agent      Dayton’s          interpretations          are
    inconsistent with each other. For example, she testified that
    the use of “2” by the conspirators means either $200, J.A. 600,
    or    $2,000,     J.A.     533.     In     fact,          in   the    explanation       for
    interpreting “2” to mean “$2,000,” Agent Dayton testified that
    drug traffickers “drop zeros [sic] to make the numbers sound
    smaller.” 
    Id. This would
    be an explanation for both of these
    interpretations, and there is no indication in the record why
    Agent     Dayton’s      expert     methodology            reasonably      leads   her    to
    conclude that the same term means $200 in one instance, but
    $2,000 in another.
    31
    A like inconsistency is seen with regard to the number “5”:
    Agent Dayton first interprets this as “$5,000,” J.A. 854, and in
    another instance as 500 grams of heroin, J.A. 925. While there
    might    very     well    be   an     explanation      for    this,      based    on    Agent
    Dayton’s expertise, she did not explain it at trial to the jury,
    and it is not evident from the record what that explanation
    would be.
    In another instance, Agent Dayton testified that, in her
    expert opinion, “590” was code for 590 grams of heroin. J.A.
    774. She gave no explanation for the basis of this opinion, and
    importantly, her testimony that the number 590 actually meant
    590 is inconsistent with the only methodology she offered for
    how she decoded the numbers heard in the calls: that the dealers
    spoke in code about numbers by dropping the zeroes.
    In   several        instances,        Agent    Dayton    “decoded”         words    and
    phrases    that     needed       no   expert       translation      at    all    since   the
    meaning     was    either      apparent        on    its   face     or     apparent      with
    contextual        information         that     any    fact     witness          could    have
    provided.        See, e.g., J.A. 406-09 (testimony that the phrases
    “first one” and “second one” were code for different deliveries
    of   heroin);      J.A.    410,       412    (testimony      that    the    phrase      “over
    there”     was    code     for      Baltimore);       J.A.    854,       857,    875,    1190
    (testimony that the word “stuff” was code for heroin); J.A. 940
    (testimony that the word “number” was code for price in the
    32
    phrase “the number they gave me was too high.                    We’re, we’re on
    the number he told me.”); J.A. 958 (testimony that the word
    “number” was code for price in the phrase “The important thing
    is the quality and a good number.”)                  This purported “decoding”
    of language that did not actually need decoding casts further
    doubt   on     whether    Agent       Dayton   was     reliably     applying      her
    methodology. Unlike in Wilson, where we found that the expert
    “applied      his    methods    and    principles      reliably     in    the    vast
    majority of his 
    testimony,” 484 F.3d at 277
    , we cannot say the
    same is true of the record before us here. The record is replete
    with instances of Agent Dayton providing no explanation for her
    interpretation, other than a token reference to her expertise in
    the Government’s framing of questions. While the district court
    was appropriately careful in its initial examination of Agent
    Dayton’s qualifications to testify as an expert, it failed to
    maintain its “gatekeeper” role throughout that testimony, Gen.
    Elec.   Co.     v.    Joiner,    
    522 U.S. 136
    ,       142   (1997),    and    the
    Government     did    little,   if     anything,     to   protect   the    generous
    ruling it had obtained from the district court from morphing
    into error.
    III.
    The Government contends that even if it was error for the
    district court to admit Agent Dayton’s testimony as it did, the
    error was harmless because there was enough evidence outside of
    33
    Agent Dayton’s testimony to justify the jury’s conviction of
    Garcia on all five counts. We disagree with the Government’s
    harmlessness metric.
    When      nonconstitutional          error    has   been      established      in   a
    criminal case, “the Government must demonstrate that the error
    did not have a ‘substantial and injurious effect or influence in
    determining the jury's verdict.’”                   United States v. Curbelo, 
    343 F.3d 273
    ,     278     (4th     Cir.     2003)   (quoting    Kotteakos        v.   United
    States,      
    328 U.S. 750
    ,    776    (1946)).    The    Government       points     to
    other evidence presented at trial, relying on the principle that
    “where       there      is    a     significant       amount    of     evidence       which
    inculpates a defendant independent of the erroneous testimony,
    the    error     is    considered        harmless.”    
    Johnson, 617 F.3d at 295
    (citing United States v. Banks, 
    482 F.3d 733
    , 741-42 (4th Cir.
    2007)). The Government’s reliance on the dictum from Johnson, in
    which       we   refused     to     find    the    error   harmless,       is   seriously
    misplaced. 12
    12
    In Johnson, the prosecution persuaded the district court
    to admit the decoding expert’s opinion testimony as lay opinion
    evidence under Rule 701. We found error in that 
    ruling, 617 F.3d at 292-93
    , and we declined to accept the prosecution’s fallback
    contention that the testimony could have been admitted as expert
    opinion evidence under Rule 702, 
    id. at 294-95,
    and we declined
    to find that the error was harmless. The case at bar is, like
    Johnson, but another example of the vagaries in theoretical,
    methodological, and foundational challenges such testimony
    engenders and why, when not properly managed, there is an
    increased likelihood of error, perhaps harmless in some cases,
    (Continued)
    34
    Indeed, just as in Johnson, the Government’s case here is
    not ironclad. The Government first points to the testimony of
    Rodriguez,   who    testified    that       he    regularly       sold    heroin    to
    Garcia,   interpreted     certain       terms,       and    no     doubt     was    an
    impressive   witness.     Rodriguez’s            credibility       was     put     into
    question, however, not least because his testimony was in return
    for sentencing considerations by the Government in a New York
    prosecution in which he faced a maximum potential sentence of
    life in prison and, in addition or alternatively, deportation to
    the   Dominican    Republic,    perhaps      leaving       behind    his    American
    citizen spouse. Of course, the jury was unquestionably entitled
    to credit the testimony of Rodriguez, every bit as much as the
    Government   was     entitled     to        call     him     as     its     witness.
    Nevertheless, Rodriguez’s testimony required searching scrutiny,
    for, as the prosecutor explained to the jury in its closing
    argument, “when you’re trying the devil, sometimes you’ve got to
    go to hell to get your witnesses.” J.A. 1735.
    The Government also points to police surveillance of the
    coconspirators,     in   Baltimore      and        elsewhere,       but    save    one
    perhaps not in others. Cf. Galloway, --- F.3d at ---, 
    2014 WL 1424939
    at *4-6 (affirming convictions after admission of
    decoding expert testimony upon plain error review); United
    States v. Hassan, 
    742 F.3d 104
    , 135 (4th Cir. 2014) (affirming
    after finding proper admission of lay opinion testimony).
    35
    instance when Garcia was seized while directly involved with
    drugs in April 2009, these observations hold no determinative
    weight absent the addition of the illuminating wiretap calls and
    the coded language referring to drugs. Each of the other counts
    for    possession,     and     Garcia’s    involvement         in   the   conspiracy,
    rests on the connection between various individuals that was
    only established through the hundreds of calls played at trial.
    And    while   the    contents     and    context    of   many      of    these    calls
    indisputably point to illegal activity, we are unable to hold
    that    the    jury    was    unaffected       by   Agent      Dayton’s     unadorned
    interpretations.
    These fundamental flaws are exemplified in the very piece
    of    evidence   the   Government        points     to   against      Garcia      in   its
    harmlessness argument: Feliciano’s call to Garcia after drugs
    were seized from her bags at a travel plaza. In testifying about
    that call, between Feliciano and Garcia, Agent Dayton opined
    that the term “stuff” was “code for heroin.” J.A. 1190. As with
    so much of Agent Dayton’s testimony, no foundation was laid, and
    there was no explanation for the coding interpretation, leaving
    us to speculate that perhaps the basis of her opinion was the
    fact that the agents had just seized 500 grams of heroin from
    Feliciano.     Without       the   remotest    effort     by    the   Government       to
    justify such testimony under Rule 702, Agent Dayton’s “expert
    opinion” was little more than an expert veneer glossed on an
    36
    item of evidence, e.g., the meaning of the term “stuff,” that
    the jury was eminently able, if it so chose, to find the meaning
    of for itself. But the jury was deprived of an opportunity to
    put to use its commonsense, mature judgment by the Government’s
    gilding of the expert witness lily.
    Even if there was more uncompromised evidence, as Garcia
    points out, this Court’s inquiry is not “‘merely whether there
    was   enough   [evidence]   to    support     the   result,   apart      from     the
    phase affected by the error. It is rather, even so, whether the
    error itself had substantial influence.’” 
    Curbelo, 343 F.3d at 286
      (quoting   
    Kotteakos, 328 U.S. at 765
    ).   Here,   in     a    trial
    spanning twelve days, Agent Dayton testified on six different
    days, recalled to the stand eighteen times. From the beginning
    of the trial to the end of the trial, the calls and the meaning
    of the words used in those calls were the centerpiece of the
    Government’s case. There was little direct evidence connecting
    Garcia to three of the four actual possession charges, and law
    enforcement never observed Garcia actually exchanging drugs or
    money with any coconspirators. We cannot find Agent Dayton’s
    testimony harmless under the circumstances.
    IV.
    In Wilson, we criticized defense counsel for failing to
    react   aggressively   to     nudge     the    district     court     to       better
    exercise its “gatekeeping” responsibilities in respect to a law
    37
    enforcement witness decoding expert. 
    Wilson, 484 F.3d at 278
    n.5
    (“Appellants deserve some of the blame for those rare instances
    where improper testimony slipped through the gate’s cracks.”).
    Here,    no   such   scolding     is    appropriate.         Garcia    timely    and
    repeatedly objected regarding the foundational sufficiency and
    methodological reliability of the agent’s expert testimony, and
    he specifically pointed to the risk of prejudice arising from
    the agent’s dual capacity as both an expert and fact witness.
    Counsel objected early and often, always respectfully and, on
    occasion,     with   success.     But   even       when    the     district     court
    sustained     some   objections     (after     a     bench       conference),     the
    Government would often “move on” to its next question or its
    next area of interest, leaving prejudicial effects hanging in
    the air. Although here, as usual, the quantum of prejudice is
    not   susceptible    of   exact   measurement,        we   are     persuaded     that
    Garcia has established his entitlement to relief.
    For the reasons set forth above, the judgment is vacated
    and     the   case   is    remanded      for       further       proceedings      not
    inconsistent with this opinion.
    VACATED AND REMANDED
    38