WASHINGTON — Senate committees investigating Russia's interference in the 2016 election are combing through witness testimony for possible misleading or untruthful statements, according to three people familiar with the effort.

The review of testimony to Senate intelligence and judiciary committees comes as President Donald Trump's former personal lawyer, Michael Cohen, pleaded guilty Thursday to a charge brought by special counsel Robert Mueller that he lied to Congress to cover up efforts during the presidential campaign to build a Trump Tower in Moscow.

The top Democrat on the Senate Intelligence Committee, Sen. Mark Warner of Virginia, said Thursday that the committee had made multiple criminal referrals to Mueller, but added "we're not going to talk about any individuals."

Sen. Susan Collins, R-Maine, said the intelligence committee "made referrals where appropriate. I am very glad the special counsel is pursuing those who mislead members of congress."

The committee's chairman, Sen. Richard Burr, R-N.C., had a stern warning Thursday for witnesses appearing before Congress.

"This is a reason people shouldn't lie when they're in front of a congressional investigation," Burr said.

Sen. John Cornyn, R-Texas, who serves on the judiciary committee, said, "I wish we lived in a world where people always told the truth but that's not necessarily the case." He said he didn't want to "speculate" about who specifically may have been untruthful before the committee.

Sen. Marco Rubio, R-Fla., said it was "possible" that other witnesses had lied. "I think there probably will be some efforts to reinterview some people as a result of what happened today."

Some Democrats on the Senate intelligence and judiciary committees, including Chris Coons of Delaware and Richard Blumenthal of Connecticut, have said publicly that they suspect that some witnesses, including the oldest president's son, may have given misleading or false statements in testimony.

"My impression from watching Donald Trump Jr. in that meeting is that he evaded and contradicted himself in many of his answers," Blumenthal said on the day the transcript was released. "I have no confidence that he has told the whole truth."

Trump Jr.'s lawyer has denied the allegation.

Donald Trump Jr. and Kimberly Guilfoyle arrive ahead of President Donald Trump and first lady Melania Trump for the lighting of the National Christmas Tree lighting ceremony at the Ellipse near the White House on Nov. 28, 2018. Andrew Harnik / AP

At least one witness, longtime Trump associate Roger Stone, has amended his original testimony.

More than 200 witnesses have appeared before the Senate Intelligence Committee alone as it seeks to learn about Russian interference in the 2016 election.

The special counsel does have access to a select number of transcripts from witness interviews conducted by the Senate Intelligence Committee, though Warner would not specify which ones or how many.

Mueller's office reviewed the transcript of Cohen's testimony with the consent of Cohen's lawyer, according to a Senate Intelligence Committee official familiar with the process.

Republicans on the House Intelligence Committee blocked a Democratic effort in September to make all of its interview materials available to the special counsel. But most of those interview transcripts will be released publicly in a redacted state as early as December.

Democrats on the committee tell NBC News that one of the committee's first acts in the new Congress will be to send the special counsel all unredacted transcripts of interviews the panel conducted in 2017 and 2018.

Rep. Adam Schiff, D-Calif., the incoming chairman of the House Intelligence Committee, said Thursday that his panel intends in the new Congress to look at "witnesses we think were untruthful before our committee and committed perjury." He specifically cited Stone as one witness for whom he has "serious questions about whether he committed perjury."

Cohen's guilty plea to a single count of making false statements to Congress about discussions, including with Trump, about a building project in Moscow was the latest twist in a special counsel investigation that has resulted in charges against 33 people.

Federal prosecutors said Cohen lied in order to minimize links between Trump and a possible Trump Tower in Moscow and to obscure the timing of the discussions about it.

Trump responded by accusing Cohen of "lying about a project that everybody knew about."