"To date, we have not seen fraud on a scale that could have affected a different outcome in the election," said the top US law enforcement agent.
His comments are seen as a big blow to Mr Trump, who has not accepted defeat.
He and his campaign have filed lawsuits in states that he lost, as they begin certifying Joe Biden as the winner.
Since 3 November's vote, Mr Trump has repeatedly made unsubstantiated claims of widespread voter fraud, and members of his legal defence team have spoken of an alleged international plot to hand Mr Biden the win.
