Donald Trump has hit back angrily over new claims about his former national security adviser Michael Flynn, who resigned amid controversy over his links with Russia.
Former acting attorney general Sally Yates spoke publicly for the first time in Monday, saying she had warned the White House in January about Mr Flynn.
She said he "essentially could be blackmailed" by the Russians because he apparently had lied to his bosses about his contacts with Moscow's ambassador in Washington.
Ms Yates was appointed by the Obama administration and was fired by Mr Trump on January 30 for refusing to defend his travel ban.
She appeared before a Senate panel investigating Russian interference in the 2016 presidential election, and told of her discussions with White House counsel Don McGahn.
She said she had warned him that Mr Flynn had apparently misled the administration about his communications with Sergey Kislyak, the Russian ambassador.
White House officials, including Vice President Mike Pence, had insisted that Mr Flynn did not discuss US-imposed sanctions with Mr Kislyak during the presidential transition period.
But they asked Mr Flynn to resign after news reports indicated he had lied about the nature of the calls.
Ms Yates said: "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,"
She added: "To state the obvious, you don't want your national security adviser compromised with the Russians."
Mr Trump has repeatedly said he has no ties to Moscow and is not aware of any involvement by his aides in any Russian interference in the election.
He has 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.
After the hearing, Mr Trump tweeted: "The Russia-Trump collusion story is a total hoax, when will this taxpayer funded charade end?"
He also tweeted: "Sally Yates made the fake media extremely unhappy today - she said nothing but old news!"
Before the hearing, the president had tweeted that it was the Obama administration, not he, that had given Mr Flynn "the highest security clearance" when he worked at the Pentagon.
Mr Trump made no mention of the fact that Mr Flynn had been fired from his position by the Obama administration in 2014.
Separately on Monday, former Obama officials said he had raised general concerns about Mr Flynn with Mr Trump and had told the incoming president there were better people for the national security post.
Mr Trump's press secretary Sean Spicer said in response that if Mr Obama "was seriously concerned" about Mr Flynn's connections to Russia or other foreign countries, he should have withheld Mr Flynn's security clearance.
The hearing on 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.