US Attorney-General William Barr has told federal prosecutors to look into possible "substantial" allegations of voting irregularities in last week's election but urged them not to pursue "fanciful or far-fetched claims".
The letter from Barr followed days of attacks on the integrity and legality of the election by President Donald Trump and his Republican allies, who have alleged widespread voter fraud without evidence.
Trump has not conceded the election to Joe Biden who on Saturday secured more than 270 votes in the Electoral College to capture the presidency.
Earlier on Monday, Trump's campaign filed a lawsuit to block Pennsylvania officials certifying Biden's victory there.
"I authorize you to pursue substantial allegations of voting and vote tabulation irregularities prior to the certification of elections in your jurisdictions in certain cases, as I have already done in specific instances," Barr wrote in the letter to federal prosecutors and the FBI director.
He said nothing in his letter should be read to indicate the Justice Department had in fact uncovered voting irregularities that affected the election outcome.
The letter was the first time Barr had addressed claims of voter fraud since last Tuesday's showdown.
"While serious allegations should be handled with great care, specious, speculative, fanciful or far-fetched claims should not be a basis for initiating federal inquiries," Barr wrote.
Democrats and the Biden campaign said Barr was fuelling the very far-fetched claims he claimed he was guarding against.
"Those are the very kind of claims the president and his lawyers are making unsuccessfully every day, as their lawsuits are laughed out of one court after another," said Bob Bauer, a senior adviser to Biden.
On Monday, the Trump campaign and two registered voters brought a lawsuit in federal court alleging Pennsylvania's mail-in voting system violated the US Constitution by creating "an illegal two-tiered voting system" where voting in person was subject to more oversight than voting by mail.
It was filed against Pennsylvania Secretary of State Kathy Boockvar and the boards of elections in Democratic-leaning counties that include Philadelphia and Pittsburgh.
The Trump campaign has filed several lawsuits since claiming the election results were flawed.
Judges have tossed lawsuits in Michigan and Georgia and experts say the Trump legal efforts have little chance of changing the outcome of the election.
Los Angeles law professor Jessica Levinson said the latest suit was unlikely to succeed and "reads like a rehash of many of the arguments the Trump legal team has made".
Also on Monday, some Republican state legislators in Pennsylvania said they would "call for a legislative-led audit of the 2020 election and demand election results not be certified, nor electors be seated, until the audit is complete".
A candidate becomes president by securing the most electoral votes rather than winning a majority of the popular vote.
Electors generally cast votes for the winner of the popular vote in their respective states.
They are slated to meet on December 14.
Barr's letter came several hours after he met with Republican Senate Majority Leader Mitch McConnell, who said earlier on Monday Trump was well within his rights to look into claims of "irregularities".