The following excerpt is from Bisby v. Zenon, 19 F.3d 1439 (9th Cir. 1994):
It is improper for the prosecution to vouch for the credibility of a government witness. Vouching may occur in two ways: the prosecution may place the prestige of the government behind the witness or may indicate that information not presented to the jury supports the witness's testimony. The first type of vouching involves personal assurances of a witness's veracity....
United States v. Roberts, 618 F.2d 530, 533 (9th Cir.1980) (citations omitted).
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