President Donald Trump on Wednesday morning doubled down on his repeated claims that voter fraud was widespread in November's election, calling for a "major investigation."
Trump made the announcement on Twitter shortly after 7 a.m., building on his previous claims that voting by illegal immigrants cost him the popular vote.
Trump has repeatedly claimed that the 2016 election was tainted by massive voter fraud.
Trump's pledge to call for an investigation comes after he told members of Congress on Monday at a private White House reception that he believes he lost the popular vote in his election because 3 million to 5 million undocumented immigrants cast votes for his opponent, his press secretary said Tuesday.
Hillary Clinton won the popular vote by nearly 2.9 million votes, but Trump won enough states to secure 306 Electoral College votes and the presidency. Since his election, he has repeatedly said he would have won the popular vote if not for massive voter fraud that benefited Clinton.
Trump's team has not yet provided specific evidence to back up his belief.
Asked by reporters Tuesday why the Trump administration had not ordered an investigation into voting practices, White House spokesman Sean Spicer at first said the president was "comfortable" with his victory.
"Maybe we will" investigate, Spicer said later. "We'll see where we go from here but right now the president's focus is on putting people back to work."
Trump's claim has been disputed by Democratic and some Republican officials, including those who run state election systems. Other Republicans support his claims.
Some who disagree with Trump say his allegations undermine confidence in U.S. democracy.
Previous probes by academic researchers, the Department of Justice, and other government agencies have found little evidence of large-scale voter fraud in the U.S. The National Association of Secretaries of State said there was no evidence of such fraud in 2016.
"We are not aware of any evidence that supports the voter fraud claims made by President Trump," the group of state elections officials said Tuesday in a statement. "In the lead up to the November 2016 election, secretaries of state expressed their confidence in the systemic integrity of our election process as a bipartisan group, and they stand behind that statement today."
Trump's own legal team argued that the 2016 election "was not tainted by fraud" in response to a recount effort by Green Party candidate Jill Stein in Michigan.
"All available evidence suggests that the 2016 general election was not tainted by fraud or mistake," Trump's lawyers said in a Dec. 1 legal filing.
Ohio Secretary of State Jon Husted, a Republican, said in a reply to Trump's tweet that it would be difficult for illegal voters to cast ballots.
"We conducted a review 4 years ago in Ohio & already have a statewide review of 2016 election underway," he said in a tweet Wednesday. "Easy to vote, hard to cheat."
Speaking to reporters Tuesday, Spicer referred to a study "that came out of Pew in 2008 that showed 14 percent of people who have voted were non-citizens" as evidence that 2016 voter fraud was widespread.
It was not clear what information Spicer was referring to. A Pew Charitable Trusts spokeswoman said that no such study exists.
"We did not publish a report in 2008 on that topic," Pew spokeswoman Kelly Hoffman said in an email to Bloomberg News. "Our work has focused on inefficient and inaccurate voter registration processes, which are not evidence of fraud at polling places."
Trump has previously cited a separate study from 2014 that found that 14 percent of non-citizens may have been registered to vote in 2008.
The study, first described in a 2014 Washington Post opinion piece, found that some of those non-citizens may have voted. The newspaper, at the time, published a series of rebuttals questioning the data and conclusions and has since posted a note on it saying that another peer-reviewed article argued the findings "were biased and that the authors' data do not provide evidence of non-citizen voting in U.S. elections."
A 2012 study by Pew found that as many as one in eight voter registrations in the U.S. either had significant inaccuracies or were no longer valid. The author of that study, David Becker, said the research didn't back up Trump's claim of vote fraud.
"As I've noted before, voting integrity better in this election than ever before," Becker, now the executive director of the Center for Election Innovation & Research, said Tuesday in a Twitter post. "Zero evidence of fraud."
Matthew Miller, former director of the Department of Justice's Office of Public Affairs during the Obama administration, called Trump's planned investigation "dangerous."
"The federal government is now going to launch an investigation into something where there is no evidence any wrongdoing has occurred, all because the president's ego is hurt," he said. "Then Republicans will use that investigation to justify restrictions that make it harder for people to vote."
The Associated Press and Bloomberg News contributed to this report.