On 8 May 2017, former acting Attorney General Sally Yates testified that she warned the White House on several occasions that former national security adviser Michael Flynn was could be blackmailed by the Kremlin, creating a national security vulnerability for the United States, and that Flynn had been questioned by the Federal Bureau of Investigation. Further, she said, despite her warnings, Flynn remained in his position for eighteen more days.
Yates and former Director of National Intelligence James Clapper testified before a Senate Judiciary subcommittee hearing about Flynn's communications with Russian officials, even as NBC News revealed President Donald Trump's predecessor Barack Obama had personally warned the sitting president not to hire Flynn.
Yates's and Clapper's full testimony can be seen here:
Yates testified that she had two in-person meetings and one phone call conversation with White House Counsel Don McGahn, the first on 26 January 2017 after it became clear from statements quoted in the news media that Vice President Mike Pence had been misled by Flynn about whether Flynn had spoken with Russian ambassador Sergey Kislyak. Yates said this discrepancy left Flynn open to blackmail, because the Kremlin knew he had misled his superiors and could use the information against him.
She also testified that she told McGahn that Flynn had been interviewed by the FBI at the White House on 24 January 2017, and that she had discussed with McGahn the possibility of criminal charges being leveled against Flynn, saying: "To state the obvious, you don't want your national security adviser compromised with the Russians."
Nevertheless, Flynn was only forced out of his post eighteen days after Yates told McGahn about discrepancies between what she knew to be true and in what Flynn was apparently telling Pence. During that time, Sen. Dick Durbin (D-Illinois) noted that Flynn took part in a number of classified meetings on matters of national security and hired staff, while Sen. Sheldon Whitehouse (D-Rhode Island) pointed out that Flynn sat in on a phone call between presidents Putin and Trump. Clapper said Flynn's continued employment created a potential risk, saying "There’s certainly a potential vulnerability. There’s no question about it."
Yates was fired from the Department of Justice by President Trump on 30 January 2017, after she refused to defend a now-defunct executive order barring travel to the United States from predominantly-Muslim countries. During the 8 May 2017 hearing, Yates defended her actions under intense questioning by Republican lawmakers by saying the president's order (which was halted by a federal judge) was unconstitutional, and as head of the nation's top law enforcement agency, she was obligated to uphold the law. Both Clapper and Yates denied leaking classified information to the press.
In mid-February 2017, White House Press Secretary Sean Spicer told reporters that the president was aware that Flynn had misled Pence, and that Trump was briefed on the matter the same day Yates warned the White House.