Former Acting-Attorney General Sally Yates testified before the Senate last Monday, May 8. In her testimony, she claimed she alerted the White House earlier this year that former national security adviser Michael Flynn could be “essentially blackmailed by the Russians.”

“We believed that Gen. Flynn was compromised with respect to the Russians,” Yates told a Senate judiciary subcommittee. Yates told the panel that she had a meeting with White House Counsel Donald McGahn Jan. 26 to tell him that she had information that statements made by Vice President Mike Pence, based on his conversations with Flynn, were false.

“We weren’t the only ones that knew all of this, that the Russians also knew about what General Flynn had done and the Russians also knew that General Flynn had misled the vice president and others,” Yates said, relating the contents of her conversation with McGahn.

But President Trump did not immediately fire Flynn over the apparent lie or the susceptibility to blackmail. Instead, Flynn remained in office for 18 more days. Only after the news of his false statements broke publicly did he lose his job Feb. 13.

Yates’s testimony, along with a separate revelation Monday that President Barack Obama had warned Trump not to hire Flynn, offered a more complete public account of Flynn’s stunning fall from one of the nation’s most important security posts.

It also raised fresh doubts about Trump’s judgment in keeping Flynn in place despite serious Justice Department concerns. White House officials have not fully explained why they waited so long.

At the heart of Monday’s testimony were Flynn’s conversations with the Russian ambassador to the United States, Sergey Kislyak. Flynn denied that they had discussed American sanctions, an assertion echoed by Vice President Mike Pence and the White House press secretary, Sean Spicer.

But senior F.B.I. and Justice Department officials knew otherwise. Kislyak, like many foreign diplomats, was under routine surveillance, and his conversations with Flynn were recorded, officials have said. Investigators knew that Flynn had, in fact, discussed sanctions.

On Jan. 26, Yates said, she called the White House counsel regarding “a very sensitive matter” that they could discuss only in person. Later that day, at the White House, she warned McGahn that White House officials were making statements “that we knew not to be the truth.”

Yates said she explained to McGahn how she knew Flynn’s statements were untrue, though she did not go into details Monday, citing concerns about sensitive information.

“Why does it matter to D.O.J. if one White House official lies to another White House official?” McGahn asked at a second meeting the next day, according to Yates.

It was not just a political concern, Yates replied. Intelligence services constantly look for leverage against foreign officials. If Flynn lied to his bosses, and Russian officials knew it, Moscow could use it as leverage against him.

“This is a classic technique they would use going back to the Soviet era,” said James R. Clapper Jr., the former director of national intelligence, who testified alongside Yates.

Mr. Trump blamed Obama officials on Monday, noting on Twitter that it was his predecessor’s administration that gave Flynn a security clearance.

“General Flynn was given the highest security clearance by the Obama Administration — but the Fake News seldom likes talking about that,” Trump wrote.

Obama fired Flynn from his defense intelligence job. And two days after the election, he warned Trump against making Flynn his national security adviser, two former Obama administration officials said on Monday. Obama said he had profound concerns about Flynn’s taking such a job.

“If President Obama was truly concerned about General Flynn, why didn’t they suspend his security clearance, which they approved just months earlier?” White House Press Secretary Sean Spicer asked during his daily press briefing.

But Spicer’s comments also called into question the Trump transition team’s own assessment of Flynn. Gov. Chris Christie of New Jersey, who led the transition in the days after the election, wanted Flynn to be slotted as director of national intelligence, a cabinet-level job but one with narrower responsibilities. Christie had reservations about Flynn that he shared with Trump, according to three people close to the transition.