US Attorney General William Barr told federal prosecutors on Monday to look into "substantial" allegations of voting irregularities but urged them not to pursue "fanciful or far-fetched claims."
Earlier in the day, President Donald Trump's campaign filed a lawsuit to block Pennsylvania officials from certifying President-elect Joe Biden's state victory.
"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.
It is the first time Barr has addressed claims of voter fraud since the Nov. 3 election between Trump and Democrat Joe Biden, who captured the presidency on Saturday.
Trump has repeatedly claimed, without evidence, that there was widespread voter fraud.
"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.
He said nothing in his letter should be read to indicate the Justice Department had in fact uncovered voting irregularities that impacted the outcome of the election.
Trump has not conceded the election to Biden two days after the Democrat secured the 270 votes in the Electoral College needed to win, and the Trump campaign and Republicans have brought numerous lawsuits over alleged election irregularities. Judges have already tossed cases in Georgia and Michigan.
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. Boockvar's office did not immediately respond to a request for comment.
Separately, 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."
In the United States, a candidate becomes president by securing the most “electoral” votes rather than winning a majority of the national popular vote. Electors generally cast their vote for the winner of the popular vote in their respective states. They are slated to meet on Dec. 14.
The Pennsylvania case was assigned to US District Judge Matthew Brann, an appointee of former President Barack Obama.
Barr's letter came several hours after he met with Republican Senate Majority Leader Mitch McConnell, who earlier on Monday said Trump was well within his rights to look into claims of “irregularities” in last week’s election.