The prosecution improperly bolstered its key witnesses by personally asserting the credibility of the government agents. It is impermissible per se for a prosecutor to offer personal assurances to the jury that government witnesses are telling the truth or to tell the jury that law enforcement witnesses should be believed simply because they were doing their job. The remarks affected the defendant's substantial rights and seriously affected the fairness, integrity, and public reputation of his trial. U.S. v. Garcia, 07-40245.