As she explained to a Senate Judiciary subcommittee on Monday, Flynn's falsehoods "created a compromise situation, a situation where the national security adviser essentially could be blackmailed by the Russians".
Yates testified at a Senate Judiciary subcommittee hearing that focused primarily on Flynn, and did not shed much light on other aspects of investigations of allegations that Russian Federation meddled in the 2016 USA election and whether there was collusion between President Donald Trump's campaign and Moscow.
But on Monday afternoon and evening, he posted a number of tweets commenting on the congressional hearing where Yates testified, along with former national intelligence director James Clapper.
But he went on to say that there was no evidence of collusion between the Trump campaign and the Russian government. Clapper also said he was unaware there was an FBI investigation into Russian collusion until FBI Director James Comey announced it in March.
The hearing took place hours after former Obama administration officials revealed that the outgoing president had warned Mr Trump against hiring Mr Flynn during an Oval Office meeting after the 2016 election.
Spicer described her visit to the White House to warn of Flynn as a "heads-up".
US Senator Edward J. Markey predicted that Senate Republicans would not pass the health care bill that made it through the House last week, the Globe's Priyanka Dayal McCluskey reports.
But if Trump's quick and dismissive reaction to her appearance is any guide, the Yates testimony put another dent in the administration's defenses. "Biggest story today between Clapper & Yates is on surveillance". However, Yates, who has been with the DOJ for several years, had quick answers for Cruz's questions, drawing praise from liberals.
The Senate committee's investigation can not not be wrapped up before the Federal Bureau of Investigation's ongoing probe into alleged collusion between Trump campaign and Russian Federation is completed.
According to Spicer, Flynn was not removed from his duties immediately because of "an element of due process" after the White House "was informed of [Yates] giving us a heads up". Meanwhile, it took 18 days for the White House to fire Flynn after Yates first told McGahn about his Federal Bureau of Investigation interview. The Russians knew that Flynn "lied"?
Yates said she read about the order in the newspaper. Trump's detractors say it is beyond doubt that Muslims were the ban's intended target, but the administration says it is motivated strictly by national security concerns, an area where USA presidents have wide powers.