President Donald Trump has promised a "major investigation into voter fraud", after making claims about millions of illegal ballots.
The new president said the inquiry would include "those registered to vote in two states, those who are illegal".
Mr Trump also said the probe would focus on "those registered to vote who are dead (and many for a long time)".
He has alleged that up to five million illegal votes were cast for Hillary Clinton, but has offered no proof.
Mrs Clinton, the Democratic candidate, received nearly three million votes more than Mr Trump, who won the presidency by prevailing in so-called swing states.
Is there any evidence?
- Unsubstantiated claim was started by self-styled conservative voter fraud specialist Greg Phillips, who tweeted: "Number of non-citizen votes exceeds 3 million"
- His tweets were picked up by right-wing fringe websites such as Infowars.com
- Fact-checking website Snopes.com says there is "zero evidence" that "illegal aliens" voted in election
- "Don't buy it," says Politifact, which points to research suggesting there have been 56 cases of non-citizens voting numbers between 2000-2011
- A Pew study in 2012 found millions of invalid voter registrations because people moved or died, but "zero evidence" of fraud
"Depending on results, we will strengthen up voting procedures!" Mr Trump said of his planned inquiry.
Mr Trump is later expected to sign immigration-related decrees, including on his plan to build a wall along the US border with Mexico.
The president first made the claim about voter fraud in a late November tweet.
Fact-checkers have rejected it as untrue and Republican election officials in key states have said they found no proof of fraudulent voting.
White House press secretary Sean Spicer said on Tuesday that Mr Trump continues to believe it, "based on studies and evidence that people have presented to him".
Mr Spicer was repeatedly pressed to specify such research, but failed to do so.
BBC Reality Check: Did millions vote illegally?
Non-citizens of the United States, including permanent legal residents, do not have the right to vote in presidential elections.
Voter registration requires applicants to declare their citizenship status, and they could face criminal punishment if they falsely claim citizenship rights.
In addition to being registered voters, in two-thirds of states, voters are required to bring identification to the polls in order to be allowed to vote.
In all states, first-time voters who register to vote by post must provide valid identification before voting.
Although it is not impossible for non-citizens to break voting laws, there is no evidence that millions of immigrants without the right to vote influenced the outcome of the popular vote.
On Tuesday, the National Association of Secretaries of State said it had confidence in the "systemic integrity of our election process" and was not aware of any evidence related to Mr Trump's claims.
Some fellow Republicans also admonished Mr Trump and urged him to drop the matter a day after the closed doors meeting with congressional leaders.
Senator Lindsey Graham called the comments "inappropriate", adding that Mr Trump should "knock this off".
He continued that the president "seems to be obsessed with the idea that he could not have possibly lost the popular vote without cheating and fraud".
Republican Pennsylvania Representative Charlie Dent also weighed in, saying Mr Trump needed to move on and "get to the serious business of governing".
Independent Senator Bernie Sanders said it was "nonsensical" and he feared Mr Trump was paving the way for Republican governors to "go forward with voter suppression".