United States v. Moralez, 2015 U.S. App. LEXIS 21341 (8th Cir. Dec. 10, 2015):

Moralez argues that Agent Taylor's dual roles as fact and expert witness prevented effective cross-examination because attempted impeachment of his expert credentials ran the risk of eliciting, and bolstering the credibility of, otherwise inadmissible or disputed fact testimony. Although we have recognized the [*5]  propriety of admitting the expert testimony of experienced investigators to assist jurors in understanding drug jargon, e.g., United States v. Delpit, 94 F.3d 1134, 1145 (8th Cir. 1996), we have not directly addressed the risks and necessary precautions attending an investigating agent simultaneously testifying as both a fact witness and an expert.⁴ In United States v. Dukagjini, 326 F.3d 45 (2d Cir. 2003), the Second Circuit noted several risks associated with dual-role testimony: (1) the witness's aura of credibility as an expert may inflate the credibility of her perception as a fact witness in the eyes of the jury; (2) opposing counsel is limited in cross-examining the witness due to the risk that an unsuccessful attempt to impeach her expertise will collaterally bolster the credibility of her fact testimony; (3) the witness may stray between roles, moving from the application of reliable methodologies into sweeping conclusions, thus violating the strictures of Daubert⁵ and Federal Rule of Evidence 702; (4) jurors may find it difficult to segregate these roles when weighing testimony and assessing the witness's credibility; and (5) because experts may rely on and disclose hearsay for the purpose of explaining the basis of an expert opinion, there is a risk the witness may relay hearsay when switching to fact testimony. [*6]  Id. at 53-54, 56-59.

4   In United States v. Coleman, 284 F.3d 892, 894 (8th Cir. 2002), we held that it was not an abuse of discretion to allow the testimony of an undercover agent interpreting jargon "as both a witness to the statements and as a law-enforcement officer with experience in drug jargon." We did not, however, grapple with the issue of dual-role testimony so much as recognize alternative bases for foundation.

5   Daubert v. Merrell Dow Pharm., Inc., 509 U.S. 579 (1993).

Other circuits have identified steps district courts may take to ameliorate these risks. One approach is the use of cautionary instructions to the jury on how to evaluate dual-role testimony. E.g., United States v. Vera, 770 F.3d 1232, 1243 (9th Cir. 2014); United States v. Lopez-Medina, 461 F.3d 724, 745 (6th Cir. 2006). The Seventh Circuit, however, has reasoned that a contemporaneous, dual-role instruction may confuse the jury even more. United States v. Moreland, 703 F.3d 976, 983-84 (7th Cir. 2012). Standard instructions on expert testimony may be sufficient where the testimony makes the basis for the witness's answers apparent. United States v. Dodson, 450 F. App'x 505, 510-11 (6th Cir. 2011). Another approach is to monitor counsel's questioning so that the jury is aware which hat the witness is wearing. E.g., United States v. Jones, 763 F.3d 777, 803-04 (7th Cir. 2014), cert. denied, Young v. United States, 135 S. Ct. 2068 (2015); United States v. Thomas, 74 F.3d 676, 682-83 (6th Cir. 1996), abrogated on other grounds by Morales v. Am. Honda Motor Co., Inc., 151 F.3d 500, 515 & n.4 (6th Cir. 1998). Counsel's questioning can also make a clear transition between roles when laying foundation (e.g., "I'd like to shift gears here a little bit and talk about some of your education, professional training, [*7]  and law enforcement experience."). United States v. Anchrum, 590 F.3d 795, 804 (9th Cir. 2009). The witness may testify separately in each capacity. United States v. Garcia, 752 F.3d 382, 392 (4th Cir. 2014). The cases imply that the less clear the segregation of roles during testimony, the greater the mitigation required of cautionary instructions. See Lopez-Medina, 461 F.3d at 743-45.

No circuit, it should be noted, has categorically prohibited the use of dual-role testimony by case agents, and failure to take these precautions has only occasionally merited reversal. In Vera, the Ninth Circuit held the failure to instruct the jury on how to evaluate dual-role testimony in combination with the failure to lay foundation for the evidence was plain error and vacated the jury's drug-quantity findings. 770 F.3d at 1243. In Garcia, the Fourth Circuit vacated a conviction where the testifying agent offered his personal knowledge of the facts surrounding the investigation as a basis for his expertise, did not apply reliable methodologies, and often failed to state any foundation for his interpretations. 752 F.3d at 391-92. The Sixth Circuit vacated a conviction where the testimony was similarly flawed, and the district court did not instruct the jury on either dual-role or expert testimony. Lopez-Medina, 461 F.3d at 748-49.

We agree that district courts and counsel should take appropriate measures to [*8]  minimize the problems that may arise from dual-role testimony by a case agent. Ideally, the lay and expert testimony would be provided by separate witnesses. When the prosecution needs to make use of the expertise of a case agent providing lay testimony, it might consider bifurcating the questioning. As outlined above, jury instructions and careful questioning are effective tools as well. The difficulty in cross-examining a dual-role witness Moralez objects to should not, of itself, warrant the exclusion of such testimony. The issue for review is whether the questioning and jury instructions sufficiently guarded against the risks identified in Dukagjini. We conclude they did in this case. None of the errors identified in Vera, Garcia, and Lopez-Medina are present here. The prosecution made clear transitions into and out of Agent Taylor's expert testimony and consistently referenced his "experience" when asking about drug jargon.⁶ This alerted the jury to the basis for Agent Taylor's answers and demonstrated adherence to a reliable methodology⁷ In addition, the district court instructed the jury at the end of trial on how to weigh expert testimony. Affording proper deference to the district [*9]  court, we conclude it was not an abuse of discretion to admit Agent Taylor's expert testimony.

6   We note that this phrasing, though sufficient in the present case, does run the risk that a jury might interpret "experience" as experience with the case, rather than general experience as a narcotics investigator. See Jones, 763 F.3d at 803.

7   The methodology underlying the interpretation of drug jargon is "the application of extensive experience to analyze the meaning of the conversations." Fed. R. Evid. 702 advisory committee's note to 2000 amendment.