Yates described discussions with Trump White House Counsel Don McGahn in which she warned that Flynn apparently had misled the administration about his communications with Sergey Kislyak, the Russian ambassador.

White House officials had insisted that Flynn had not discussed US-imposed sanctions with Kislyak during the presidential transition period, but asked Flynn to resign after news reports indicated he had lied about the nature of the calls.

“We felt like it was critical that we get this information to the White House, in part because the vice president was unknowingly making false statements to the public and because we believed that General Flynn was compromised with respect to the Russians,” Yates said.

“To state the obvious,” she added later, “you don’t want your national security adviser compromised with the Russians.”

Yates’ questioning was just one portion of a politically charged day that began with combative tweets from Trump and continued with disclosures from Obama administration officials about a private Oval Office conversation between Obama and his successor.

Republican senators in the hearing repeatedly pressed Yates on an unrelated matter — her refusal to defend the Trump administration’s travel ban — and whether she was responsible for leaking classified information. She said she was not.

Trump shouldered into the conversation early in the morning, tweeting that it was the Obama administration, not he, that had given Lieutenant General Flynn “the highest security clearance” when he worked at the Pentagon. Trump made no mention of the fact that Flynn had been fired from his high position by the Obama administration in 2014.

Trump also called the investigation of his campaign’s possible ties to Russia’s election meddling a “taxpayer funded charade”, and the suggestion itself a “total hoax”.

Yates filled in new details of the events of January 26, describing contacting McGahn in the morning and telling him she had something sensitive to discuss in person. Later that day, at the White House, she told him there was an alarming discrepancy between how Trump officials, including Vice President Mike Pence, were characterising Flynn’s contacts with Kislyak and what intelligence officials knew to be true based on recordings of those calls.

The pair spoke several times over the next two days, with McGahn asking Yates how Flynn had fared during an interview with the FBI earlier that week — she did not answer — and why it was the business of the Justice Department if White House officials had misled each other. Flynn’s forced February resignation followed media reports that he had discussed US-imposed sanctions on Russia with Ambassador Kislyak, which was contrary to the public representations of the Trump White House.

Yates herself, a longtime federal prosecutor, was fired by Trump on January 30 after refusing to defend his travel ban. James Clapper, director of national intelligence under President Obama, testified as well on Monday. He retired when Trump took office.

Separately on Monday, former Obama officials said he had raised general concerns about Flynn with Trump and had told the incoming president there were better people for the national security post. Trump’s press secretary Sean Spicer said in response that if Obama “was seriously concerned” about Flynn’s connections to Russia or other foreign countries, he should have withheld Flynn’s security clearance. Flynn served under Obama as defense intelligence chief before Obama dismissed him.

Trump repeatedly has said he has no ties to Russia and isn’t aware of any involvement by his aides in any Russian interference in the election. He’s dismissed FBI and congressional investigations into his campaign’s possible ties to the election meddling as a hoax driven by Democrats bitter over losing the White House.

The Associated Press reported last week that one sign taken as a warning by Obama officials about Flynn’s contacts with Kislyak was a request by a member of Trump’s own transition team made to national security officials in the Obama White House for the classified CIA profile of Kislyak.

Yates’ warning about Flynn in January capped weeks of building concern among top Obama officials, former officials told the AP. Obama himself that month told one of his closest advisers that the FBI, which by then had been investigating Trump associates’ possible ties to Russia for about six months, seemed particularly focused on Flynn.

Yates had been scheduled to appear in March before the House intelligence committee, but that hearing was canceled.

The subcommittee meeting Monday is one of multiple congressional probes into the Russia interference, along with House and Senate intelligence panels. All the committees are led by Republicans.