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.
Former acting Attorney General Sally Yates - an Obama appointee who was sacked by Mr. Trump after she refused to defend an executive order that barred travel from several Muslim majority countries - told the committee that she had warned the White House that former National Security Adviser Michael Flynn may have been compromised by Russian Federation.
In the hearing, former CIA Director James Clapper and Yates were questioned about evidence of possible collusion between Trump associates and Russian Federation.
"Director Clapper reiterated what everybody, including the fake media already knows - there is "no evidence" of collusion w/ Russian Federation and Trump", Trump said.
Clapper repeated that assertion Monday, although he was unaware of an FBI investigation into the matter.
Trump fired Flynn, a retired general, for failing to disclose discussions with Russian ambassador Sergey Kislyak about USA sanctions against Moscow and for not telling the truth about them to Vice President Pence.
What happened in the 18 days between when the White House received a warning about Flynn and his firing?
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MONTANARO: Well, this all stems back to a conversation that Michael Flynn had December 29 with the Russian ambassador, could have been about those sanctions that President Obama announced that same day.
At issue is whether Flynn improperly indicated to Kislyak the Trump administration would ease or reverse economic sanctions former President Barack Obama had imposed on Moscow in retaliation for Russian interference in the USA presidential campaign. And that created a compromised situation, a situation where the national security adviser essentially could be blackmailed by the Russians. He was in charge of the FBI's investigation into Russian interference in the 2016 election, and on March 20, he testified that the investigation included looking at contacts between the Trump campaign and Russian officials.
Flynn had been pushed out by Obama in 2014 from his job as director of the Defense Intelligence Agency, or DIA.
Trump tweeted Monday that Flynn was "given the highest security clearance by the Obama Administration - but the Fake News seldom likes talking about that".
Spicer aimed to cast doubt on the credibility of Yates, a career Justice Department official who Trump fired after she declined to defend the first version of Trump's divisive restrictions on travel from several predominantly Muslim countries.
"We knew that was not a good situation, which is why we wanted to let the White House know about it".
Yates' testimony returned the spotlight to the simmering controversy over Russia's meddling in last year's United States election. This development comes after former president Barrack Obama [official website] said he had warned Trump not to hire Flynn [NYT report].