Flynn was sacked in February for lying to Vice President Mike Pence about his discussions with Russia's ambassador. Trump fired Yates Jan. 30 after she said the Justice Department would not defend the president's proposed travel ban.
Obama gave the warning in an Oval Office meeting with Trump just days after the Republican's surprise election win last November 8.
Flynn spoke with Kislyak in December, just before and after the Obama administration slapped new sanctions on Russian Federation for its cyber attacks during the presidential campaign. Yates noted that she's not at DOJ anymore, "so I don't know what's been referred".
Flynn was forced to resign after it was alleged that he discussed the sanctions imposed on Russia by President Obama in a phone conversation with Russian Ambassador Kislyak.
Needless to say, reporters are curious as to why Flynn was allowed to remain in a top national security role even after the Justice Department warned he had been compromised by the Russian government.
"The underlying conduct that Gen. Flynn had engaged in was problematic in and of itself", she testified Monday.
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She said she was briefing the Trump White House so that officials could take "the action that they deemed appropriate" and that she believed the Russians already had the same information.
Mr Trump hit back by dismissing suggestions that his team colluded with Russian Federation as a "hoax", and calling the congressional investigations into Russia's interference in the United States election a taxpayer- funded "charade".
Monday, Yates told a Senate subcommittee that, soon after the inauguration, she went to the White House counsel with an urgent warning: that retired Gen. Michael Flynn's knowing lies about his contacts with Russian Federation left him subject to manipulation by foreign adversaries.
Now this might seem confusing because Trump is blaming Obama for Trump hiring and not vetting Flynn.
The dual testimony by Yates and Clapper returned the spotlight to the simmering controversy over Russia's meddling in last year's United States election. So, Yates reached out to the White House Counsel Don McGahn, and told him that Flynn had apparently lied to Vice President Pence and others.
White House press secretary Sean Spicer on Tuesday defended the Trump administration's handling of Michael Flynn's firing amid criticism of how long it took the former national security advisor to leave the White House. During their conversation, Mr. Obama warned Mr. Trump not to hire Flynn, whom he himself fired in 2014 from his position as head of the Defense Intelligence Agency because of his temperamental issues. James Clapper, director of national intelligence under Obama, also testified Monday. If Flynn was such a threat to national security, then why was his clearance reauthorized by Team Obama?