Subject to limitations, a witness may be impeached -- the credibility of a witness may be attacked -- by evidence of untruthfulness (lack of veracity), bias and interest, inability to understand and mental disorder, inability to communicate, inability to perceive, or prior inconsistent statements.

United States v. Snow, 517 F.2d 441, 444 (9th Cir. 1975) ("We are aware, as was the trial court, that the inference sought to be drawn by the prosecution from the name tape might be false. Someone other than the....The constitutional requirement that proof of criminal guilt be shown beyond a reasonable doubt is not a requirement that proof of guilt be conclusive, completely certain, or beyond any doubt.

McDonough v. City of Quincy, 452 F.3d 8, 21 (1st Cir. 2006) (holding, in plaintiff police officer's unlawful retaliation action against defendant city, that the trial court properly admitted the out-of-court statements....A statement by an agent or an employee who is not authorized to speak on behalf of the agent's principal or the employee's employer may nonetheless be treated as the statement of the principal or the employer and thus as a vicarious admission that is exempt from the prohibition against hearsay; but such a statement can be imputed to the principal or the employer only if the statement, if by an agent, is about a matter within the scope of the agent's agency or, if by an employee, the statement is about a matter within the scope of the employee's employment.

Fed. R. Evid. 801(d)(1)(B) ("A statement is not hearsay if-- ... (1) Prior statement by witness. The declarant testifies at the trial or hearing and is subject to cross-examination concerning the statement, and the statement is ... (B) consistent with the declarant's testimony and is offered to rebut an express or implied charge against...A prior statement of a witness is exempt from the prohibition against hearsay evidence if the prior statement is consistent with the witness' testimony at the trial or hearing at which the prior statement is offered, the prior statement is offered to rebut a express or implied charge of recent fabrication or motive, and the witness who made the prior statement testifies and is subject to cross-examination at the trial or hearing at which the prior statement is offered.