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 effected a different outcome in the election," said the top US law enforcement agent.
His comments are seen as a big blow to 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.
President-elect Biden defeated the incumbent by a margin of 306 to 232 votes in the US electoral college, which chooses the US president. And in the popular vote, Biden won at least 6.2 million more votes than Trump.
Since 3 November's election, 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 Biden the win. Today, after Barr's statements were released, the president tweeted several times alluding to voter fraud, again without proof.
"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," Barr, who is seen as a top Trump ally, told AP News, referring to the assertion that ballot machines were hacked to give more votes to Biden.
Barr said that the Department of Justice (DOJ) and the Department of Homeland Security have investigated that claim, "and so far, we haven't seen anything to substantiate that".
Last month, the top lawyer issued an order to US attorneys, allowing them to pursue any "substantial allegations" of voting irregularities, before the 2020 presidential election was certified.
"There's a growing tendency to use the criminal justice system as sort of a default fix-all, and if people don't like something they want the Department of Justice to come in and 'investigate'," he added. He also told the AP that he had appointed a veteran prosecutor to continue investigating the origins of special counsel Robert Mueller's probe into alleged election meddling.
Reacting to his comments, Trump campaign lawyers Rudy Giuliani and Jenna Ellis said in a joint statement: "With the greatest respect to the Attorney General, his opinion appears to be without any knowledge or investigation of the substantial irregularities and evidence of systemic fraud."
Prediction Barr will be sacked
Meanwhile, Senate Democratic leader Chuck Schumer said of Barr's statement: "I guess he's the next one to be fired."
In Trump's first post-election TV interview on Sunday (local time), he told Fox News he would continue to pursue every legal challenge available.
"My mind will not change in six months," he said by phone, adding: "There was tremendous cheating here."
He also floated the idea of having a special counsel appointed to investigate the election. Any such special counsel would have to be approved by Barr.
Barr is not the first senior US official to declare the election free from tampering.
Chris Krebs, who headed the US Cybersecurity and Infrastructure Security Agency, was fired last month after he disputed Trump's fraud claims. The 2020 election "was the most secure in American history", he had said.
Yesterday, Trump lawyer Joe DiGenova drew swift condemnation after he called for violence against Krebs.
"Anybody who thinks the election went well," he told The Howie Carr Show podcast, "like that idiot Krebs who used to be the head of cybersecurity, that guy is a class A moron.
"He should be drawn and quartered. Taken out at dawn and shot."
That threat was condemned today in a blistering tirade from Georgia's voting systems manager.
Gabriel Sterling, a Republican, said Trump would bear ultimate responsibility for any violence that results from the election fraud claims the president had stoked.
He also said the wife of Georgia Secretary of State Brad Raffensperger has been getting "sexualised threats" on her cellphone, and a young contractor received death threats after viral internet messages falsely claimed he had been caught committing fraud.
Sterling, visibly angry at a brief press conference today, directed some of his remarks squarely at Trump.
"Stop inspiring people to commit potential acts of violence. Someone's going to get hurt, someone's going to get shot, someone's going to get killed," Sterling said.
"It has all gone too far," he added. "It has to stop."
Last week, Trump publicly labelled Raffensperger, also a Republican, an "enemy of the people."
Sterling said Trump's statement about Raffensperger "helped open the floodgates to this kind of crap".
"There are some nutballs out there who are going to take this and say, 'The president told me to do this,'" Sterling said.
Trump campaign spokesman Tim Murtaugh said it is trying to make sure "that all legal votes are counted and all illegal votes are not. No one should engage in threats or violence, and if that has happened, we condemn that fully."
- BBC / Reuters