US Attorney General William Barr says his justice department has found no proof to back President Donald Trump's claims of fraud in the 2020 election.
"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.
"There's been one assertion that would be systemic fraud and that would be the claim that machines were programmed essentially to skew the election results," Mr Barr told AP News on Tuesday, referring to the assertion that ballot machines were hacked to give more votes to Mr Biden.