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Former Acting Attorney General Sally Yates testified Monday that she told the White House that then-National Security Adviser Mike Flynn could be "blackmailed by the Russians," because he misled the vice president about his "problematic" conduct.

"We were concerned that the American people had been misled about the underlying conduct and what General Flynn had done," Yates told a Senate Judiciary subcommittee.

Yates declined to go into specifics, which she said were classified, but she essentially confirmed news media accounts about what led up to the firing of Flynn in February. Flynn misled officials, NBC News and others have reported, by saying he hadn’t discussed Obama administration sanctions on Russia, when in fact he had.

Yates said she expected the White House to act on the information she conveyed. But it was only 18 days later, after the Yates warning leaked, that the White House pushed Flynn out.

Yates said she conveyed the information to White House counsel Don McGahn on Jan. 26, two days after Flynn was interviewed by the FBI on Jan. 24. McGahn asked Yates how Flynn did in the interview, she testified, "and I declined to give him an answer to that."

Yates said she felt it was critical to get the information to the White House "in part because the vice president was unknowingly making false statements to the public," and also because Flynn was compromised, given that the Russians knew he was misleading other officials.

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Yates said McGahn asked her, "Why does it matter to the DOJ if one White House official lies to another official?" She explained that the American public was being misled, and that the Russians knew that.

"To state the obvious, you don’t want your national security adviser compromised by the Russians," Yates said she replied.

Yates made clear that the concerns she raised had solely to do with Flynn’s contacts with the Russian ambassador, and not his paid speech for a Russian state media organization or his lobbying for Turkish interests, two other issues that have brought him law enforcement scrutiny.

Asked by Sen. Richard Blumenthal (D-Connecticut) if Flynn lied to the FBI, Yates said she couldn’t answer because that would compromise an "ongoing FBI investigation." Her remarks appeared to confirm that Flynn is the target of an FBI investigation.

In other developments, former Director of National Intelligence James Clapper told senators he had not been aware of the FBI’s counter-intelligence investigation into possible collusion between Trump associates and the Russian election interference operation.

"Given its sensitivity, even the existence of a counter-intelligence investigation is closely held, including at the highest levels," Clapper said.

Former acting Attorney General Sally Yates and former Director of National Intelligence James Clapper testify on May 8, 2017, before the U.S. Senate Judiciary Committee on Capitol Hill in Washington. Jim Watson / AFP - Getty Images

This puts into context Clapper’s March 5 statement that he had seen no evidence of collusion between the Trump campaign and Russia. He repeated that statement in the hearing, saying he had seen no evidence of collusion.

But Yates declined to answer the same question, saying that her answer would require her to reveal classified information.

Yates was fired by President Donald Trump after 10 days as acting attorney general when she declined to enforce his executive order on travel and immigration.

Obama Warned Trump About Flynn

Yates and Clapper's testimony comes after NBC News revealed Monday that former President Barack Obama personally warned Trump against hiring Flynn less than 48 hours after the November election.

The president sought to get out ahead of the any unpleasant disclosures on Monday morning, casting aspersions on Yates — the the 27-year Justice Department prosecutor who warned the White House that Flynn had misled officials about his conversations with the Russian ambassador.

Michael Flynn, then, National Security Advisor to US President Donald J. Trump, attends a press conference on Feb. 10, 2017 in the East Room of the White House in Washington, D.C. Jim Lo Scalzo / EPA

"Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Counsel," Trump tweeted, referring to Yates’ conversation with White House counsel Donald McGahn.

It was more than a week after Yates raised concerns about Flynn with McGahn that the story leaked to the Washington Post, prompting a series of events that led to Flynn's ouster from his White House job.