Prosecutor’s argument regarding how the jury might find defendant guilty and vouching for the strength of his case may have constituted misconduct, but was harmless. Ruiz was convicted of federal firearm offenses. During closing argument, the prosecutor used a PowerPoint presentation that informed the jury it could find Ruiz guilty only if it found the officers lied and the witnesses were mistaken. Objections to the argument were overruled. On appeal, Ruiz in part claimed the prosecutor committed prejudicial misconduct during argument. Held: Affirmed. Witness credibility is a jury question, thus prosecutors should avoid telling the jury that if the defendant is innocent, the police must be lying. Here, where some of the evidence was susceptible of several interpretations, such argument may be an inference unsupported by the record, thus altering the burden of proof. The court found that this need not be decided, however, because any error was harmless based on the evidence and instructions given regarding the offense elements and burden of proof. With respect to Ruiz’s claim the prosecution improperly vouched for the strength of his case, the court found the majority of the argument proper. It was improper for the prosecutor to urge the jury to convict because the United States considered the evidence overwhelming, as this suggested to the jury he was providing his expert opinion regarding the strength of the government’s case. However, this improper vouching did not affect the fairness of Ruiz’s trial.