AP

After less than a month serving in Donald Trump's administration, retired Gen. Michael Flynn resigned from his post as national security adviser on Monday night, dogged by questions about his apparently cozy conversations with Russia.

In his resignation letter, Flynn admitting providing the administration "incomplete information" about the content of his call with Russian ambassador Sergey Kislyak just weeks before the inauguration in December, when Flynn was still a private citizen, about sanctions against Russia that were imposed by President Obama.

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“I inadvertently briefed the Vice President-elect and others with incomplete information regarding my phone calls with the Russian ambassador. I have sincerely apologized to the President and the Vice President, and they have accepted my apology," he wrote in the letter.

Flynn had strenuously denied speaking to the ambassador about the sanctions at all, saying their call only consisted of holiday greetings, and Vice President Mike Pence took to the cable news circuit earlier this month defending Flynn. But unnamed sources confirmed to The New York Times that Flynn indicated that relief from American sanctions could be possible under Trump, and that the pair also talked about a coordinated response to fight the Islamic State.

Justice Department officials had also warned the Trump administration that Flynn's communications with Russia could make him vulnerable to blackmail, a message that the Washington Post reported was delivering by acting Attorney General Sally Yates before she was fired late last month for refusing to enforce Trump's Muslim ban. Some officials had even indicated they were concerned Flynn's communications violated the Logan Act, an arcane law that prohibits American citizens from negotiating with foreign governments.

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For now, the White House confirmed that Lt. Gen. Joseph K. Kellogg Jr., a retired Vietnam veteran, will serve as acting national security adviser. But the news also underscored the chaos that's emerged as a dominant force in Trump's White House–Flynn's resignation came so hastily last night that at least two members of the National Security Council heard the news from media reports, according to the Times.