Can a prosecutor vouch for the truthfulness of a witness?

In United States v. Lew, 875 F.2d 219, 223-24 (9th Cir.1989), the prosecution brought out on the direct examination of two witnesses that their plea agreements required each to testify truthfully. We recognized that it was improper to allow the prosecution to elicit testimony on direct about the truthfulness requirement in a plea agreement. However, the vouching did not rise to the level of plain error because there was substantial independent evidence against the defendant, and because the judge instructed the jury to consider the extent to which the testimony of the witnesses may have been influenced by the government's promises and to look for corroborating circumstances before giving full credibility to those witnesses. Id. at 223-24.

In United States v. Simtob, 901 F.2d 799, 805 (9th Cir.1990), the prosecutor offered in front of the jury to immunize a witness for possible false statements to government officials. He then repeatedly exhorted the witness to tell the truth, and suggested during an exchange with the witness that he did not think the witness was lying. In response to defense counsel's objection, the court agreed that the prosecutor's remarks were "inappropriate" and said, "the jury will disregard." Id. at 806. The court also later instructed, "that the prosecutor cannot vouch for the truthfulness of a witness." Id. Because the case was close, we reversed applying harmless error analysis. Id.

In United States v. Monroe, 943 F.2d 1007, 1013-14 (9th Cir.1991), cert. denied, --- U.S. ----, 112 S.Ct. 1585, 118 L.Ed.2d 304 (1992), the government introduced the truthful testimony requirement of the witness's plea agreement on direct. We held that this was not vouching as it was offered in response to defense counsel's attack on the witness's credibility in his opening statement. Id. at 1014.

In United States v. Smith, 962 F.2d 923, 933-34 (9th Cir.1992), the prosecutor assured the jury in closing argument that his job was to turn over favorable evidence to the defense and to lead them to the truth, and that "[i]f I did anything wrong in this trial I wouldn't be here. The court wouldn't allow that to happen." Defense counsel had attacked a witness's credibility,

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Most recently, in United States v. Kerr, 981 F.2d 1050 (9th Cir.1992), the Assistant United States Attorney in closing argument referred to interviews he had with four witnesses and asked whether they were hoodwinking him and the court, and also said "I think ..." one witness was "very candid," and another was "candid" and "honest." The trial court gave a general instruction which did not mention the specific statements of the prosecutor and was not given immediately after the vouching occurred. We examined the closeness of the case, and thought the testimony of the four witnesses for whom the government vouched was crucial to the case and to the prosecutor's argument. We reversed for plain error. Id. at 1054.