Prosecution’s tactics of asking defendant to comment on the credibility of the key witness against him and of referring to evidence not before the jury to bolster that witness’ testimony, deprived defendant of a fair trial and warranted reversal. A Border Patrol agent apprehended Alcantara one mile from the U.S./Mexico border for illegally entering the United States. At trial, the agent testified that Alcantara confessed to being a part of a large group attempting to enter the U.S. without permission. Alcantara denied confessing and testified that he had inadvertently crossed the border in a methamphetamine-induced psychosis. Following his conviction, Alcantara appealed, arguing the prosecution engaged in improper questioning and vouching during the trial. Held: Reversed. Prosecutors must not ask defendants during cross-examination to comment on the truthfulness of other witnesses. Nor may prosecutors “vouch” for a witness by suggesting that information exists outside the record that verifies the witness’ testimony. Here, the prosecutor improperly asked Alcantara to comment on the agent’s veracity by asking if the agent was “inventing” stories, which was the equivalent of asking if the agent was lying. The government conceded that the prosecutor improperly vouched for the agent’s credibility during rebuttal by stating that the “case boils down to the credibility of a 15-year methamphetamine addict, a man who has every incentive to lie, versus the testimony and the evidence of Border Patrol agents who are sworn to uphold the law.” There was no evidence in the record that Border Patrol Agents are sworn to uphold the law. Under the federal plain error standard, the misconduct required reversal because witness credibility was paramount in this case. The trial court’s curative instruction did not sufficiently cure the error.