(CNN) - Michael Flynn has resigned as national security adviser, CNN reported Monday night.
The Justice Department warned the Trump administration last month that Michael Flynn misled administration officials regarding his communications with the Russian ambassador to the United States and was potentially vulnerable to blackmail by the Russians, a person familiar with the matter told CNN.
A White House official also confirmed the Justice Department warning.
The concern was raised after Flynn claimed he did not discuss with the Russian ambassador the sanctions being imposed by former President Barack Obama's administration in retaliation for Russia's interference in the election. Flynn was not yet in government.
The message was delivered by then-Acting Attorney General Sally Yates. Other top intelligence officials, including James Clapper and John Brennan, were in agreement the White House should be alerted about the concerns.
The Washington Post first reported the Justice Department message.
Trump fired Yates at the end of January after she told Justice Department attorneys not to defend his executive order suspending the refugee program and temporarily restricting travel to the US from seven majority-Muslim nations. Implementation of the travel ban has since been stalled by the courts and the Trump administration is weighing its options.
A representative for Yates said she no comment.
Flynn is a retired lieutenant general who was the head of the Defense Intelligence Agency under Obama until he was forced out.
He has been at the center of mounting controversy over the past several days in regard to his contacts with Russian Ambassador Sergey Kislyak. Following initial reports in early January that Flynn was in contact with Kislyak and spoke to him about US sanctions, Vice President Mike Pence offered a robust defense of the incoming national security adviser. He said in nationally televised interviews that he had spoken with Flynn and that Flynn had ensured him he had not spoken about sanctions with the Russian ambassador.
But reports of an investigation showed Flynn potentially misled Pence and criticism of the controversial former military man grew.
White House senior counselor Kellyanne Conway said on Monday the President had "full confidence" in Flynn. But about an hour later, things had changed.
White House press secretary Sean Spicer offered a much more limited commitment from the President towards Flynn, saying Trump was "evaluating the situation."
An administration official said new information came to light between Conway's comments and the Spicer statement.
Another source said Conway "did not go out there on her own" to say the President had full confidence in Flynn, and that "new information came to light in real time."
Spicer's statement said Trump was "speaking to the vice president relative to the conversation the vice president had with Gen. Flynn, and also speaking to various other people about what he considers the single most important subject there is: our national security."
Sources within the administration described a difficult situation for Flynn, with one saying "the knives are out."