Obama-era acting Attorney General Sally Q. Yates testified Monday that she told Trump administration attorneys that their national security adviser could be blackmailed by Russia, weeks before Michael Flynn resigned over concerns he lied to the White House about his foreign contacts.

Ms. Yates testified during a highly anticipated Senate Judiciary Committee subcommittee exploring suspected Russian meddling in the November presidential election. Lawmakers struggled to comprehend the timeline that led to the departure of the top White House security adviser within a month after the Trump administration took office.

The forced February resignation stemmed from reports that the retired army lieutenant-general lied to Vice President Mike Pence about pre-inauguration discussions on sanctions Mr. Flynn had with Russian Ambassador Sergey Kislyak.

On Monday, Ms. Yates told senators that she met with White House counsel Don McGahn on Jan. 26 and warned him that the Russians knew Mr. Flynn had lied to Mr. Pence. She explained that this caused a situation whereby “the national security adviser essentially could be blackmailed by Russians.”

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

Hours before Ms. Yates‘ testimony, reports emerged on NBC-TV that three former Obama administration officials, speaking anonymously, said President Obama warned Mr. Trump during an Oval Office meeting less than 48 hours after the election against hiring Mr. Flynn as national security adviser.

At the White House daily briefing, spokesman Sean Spicer confirmed the report. “It’s true that President Obama made it known that he wasn’t exactly a fan of Gen. Flynn‘s,” Mr. Spicer said.

He said it was well known that Mr. Flynn and Mr. Obama had sparred openly over the White House strategy on dealing with the Islamic State terrorist group.

Mr. Trump also took a swipe at Ms. Yates, whom he fired on Jan. 31 for refusing to defend in court his executive order for a temporary ban on travelers from seven Muslim-majority countries. The hearing Monday was her first appearance on Capitol Hill since her firing.

“Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Council,” Mr. Trump tweeted.

When asked for details of her discussions with the Trump administration about Mr. Flynn, Ms. Yates told senators she warned the White House so it could take appropriate actions.

Sen. Dianne Feinstein, California Democrat, asked Ms. Yates if Mr. Flynn had lied to the administration about his Russian contacts, particularly to Mr. Pence. Ms. Yates replied, “It certainly appeared that way — yes.”

Republicans tried to follow Mr. Trump’s advice and asked Ms. Yates about what has become a central point of dispute in the controversy: the leaking of classified information.

Since the start of the year, Democrats have argued that Mr. Trump could have colluded with the Kremlin to gain favor in the election. Republicans have argued that the real crime was the leaking of classified intelligence by Obama officials to create the impression that Mr. Trump somehow colluded with Moscow.

Ms. Yates frequently frustrated several senators who started that line of questioning, saying she could not disclose classified information. She said she could not detail where she learned about Mr. Flynn’s communications with the Russian ambassador.

Senators also asked former Director of National Intelligence James R. Clapper to explain how the intelligence community had unmasked Mr. Flynn’s identity during surveillance of Mr. Kislyak, then leaked.

Committee chairman Sen. Lindsey Graham, South Carolina Republican, specifically asked, “How did it get to The Washington Post?”

Mr. Clapper and Ms. Yates denied leaking any classified intelligence.

In January, Mr. Clapper issued a major review of Russia’s efforts to undermine the U.S. presidential election. The report went to the White House and Congress, and an unclassified version of the report was made public.

It detailed Russian cyberattacks and hacks against the Democratic National Committee and Hillary Clinton’s campaign manager John Podesta.

On Monday, Mr. Clapper told senators in his opening remarks that the Russians “must be congratulating themselves for having exceeded their wildest expectations with a minimal expenditure of resources.”

Sen. Al Franken, Minnesota Democrat, referenced the broader nature of the overall inquiry. “We’re trying to put a puzzle here together,” he said, adding that Mr. Flynn had been “a danger to this republic.”

Mr. Franken questioned Mr. Clapper about Mr. Flynn’s decision-making process: How could a former high-profile intelligence official collect an appearance fee from the Russian government?

He specifically noted that in 2015, Mr. Flynn participated in a gala anniversary celebration of Russia Today TV, commonly referred to as a Kremlin propaganda mouthpiece. For his work, Mr. Flynn received roughly $30,000, which, Mr. Franken pointed out, was after Mr. Flynn left his position as director of the Defense Intelligence Agency.

“General,” Mr. Franken asked Mr. Clapper, “since you’ve retired, have you appeared on Russia Today?”

When Mr. Clapper cracked a smile and replied, “Not wittingly, no,” the hearing broke into laughter.

Mr. Franken then targeted the Trump administration, wondering why its officials would not fire an employee who they knew lied to the vice president, was paid by the Russians, went on RT and met secretly with the Russians.

Neither Mr. Clapper nor Ms. Yates chose to comment.

• S.A. Miller contributed to this report.

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