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," he said.
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.
President-elect Biden defeated the incumbent Mr Trump by a margin of 306 to 232 votes in the US electoral college, which chooses the US president. And in the popular vote, Mr Biden won at least 6.2 million more votes than Mr Trump.
Since 3 November's election, 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. On Tuesday, after Mr 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," Mr Barr, who is seen as a top Trump ally, told AP News on Tuesday, referring to the assertion that ballot machines were hacked to give more votes to Mr Biden.
Mr 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".
A DOJ spokesperson later stressed the department had not concluded its investigation and would continue to "receive and vigorously pursue all specific and credible allegations of fraud as expeditiously as possible".
Last month, Mr Barr 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 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."
Meanwhile, Senate Democratic leader Chuck Schumer said of Mr Barr's statement: "I guess he's the next one to be fired."
It was getting late in the day for the US Justice Department to take action that would give life to Donald Trump's efforts to reverse the results of the 2020 presidential election. On Tuesday, however, Attorney General Bill Barr effectively ended even the slightest hope that federal investigators would provide the president with a political lifeline.
That the justice department did not find "fraud on a scale that could have effected a different outcome in the election" shouldn't come as a surprise. The president's own lawyers have yet to present any such evidence in their lawsuits challenging the voting results. What is noteworthy, however, is that Barr chose to comment publicly rather than stay silent and let the lack of any announced findings or charges speak for itself.
According to news reports, Trump has been complaining privately about the lack of support he's received from Barr and the FBI in his attempts to attribute his defeat to fraud. He is already feuding with Republican governors in Arizona and Georgia - two traditionally conservative states he lost to Joe Biden - for failing to echo his concerns.
Barr may be just the latest target of the president's anger.
On Tuesday, Mr Trump continued to fire off tweets alleging election fraud.
He shared a conspiracy theory about voting machines in Georgia, tweeting at the state's Republican governor: "You allowed your state to be scammed."
In other tweets he claimed Mr Biden's win in Arizona was "impossible", the results in Nevada were "fake" and also claimed without proof: "We won Michigan by a lot!"
Mr 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 Mr Trump's fraud claims. The 2020 election "was the most secure in American history," he had said.
On Monday, Trump lawyer Joe DiGenova drew swift condemnation after he called for violence against Mr Krebs.
He told The Howie Carr Show podcast "that idiot Krebs" should be "taken out at dawn and shot".
That threat was condemned on Tuesday in a blistering tirade from Georgia's voting systems manager.
What's the latest with the Trump election challenge?
On Tuesday, the president's campaign filed a lawsuit with Wisconsin's supreme court challenging the state's presidential vote results.
The petition came a day after Mr Biden was certified as the winner in the Midwestern state by about 20,000 votes.
The Trump legal action challenges more than 220,000 ballots in Wisconsin based largely on technicalities.
It alleges clerks filled out missing information on postal ballots and objects to an event where election officials collected and checked votes in a park in the city of Madison, rather than polling stations.
Mr Trump and his allies have failed to make significant headway in their efforts to challenge the results in Arizona, Pennsylvania, Georgia, Michigan and Nevada.