United States v. Rosario-Diaz
United States Court of Appeals for the First Circuit
202 F.3d 54 (1st Cir. 2000)
Ralph Rosario-Diaz and others (defendants) were charged in a fatal carjacking; co-conspirator Aponte-Lazu pleaded guilty and testified for the prosecution, and the government then called FBI agent Huff, who testified over objection about how investigators determined when Aponte-Lazu was lying versus telling the truth during the investigation, even though Aponte-Lazu's credibility hadn't been attacked by the defense. The district court instructed the jury to judge Aponte-Lazu's credibility based only on his own testimony, not Huff's, and the defendants were convicted and appealed.
Whether the prosecution may bolster a witness's credibility by making personal assurances of credibility, by indicating that facts not before the jury support the witness's testimony, or through the testimony of another witness called by the prosecution.