Michael Flynn, just recently appointed National Security Advisor by Donald Trump, has resigned his post. 

Flynn has been under heavy fire as reports surfaced that he lied to, among others, Vice President Mike Pence about the content of his phone call with the Russian ambassador. Initially, Flynn denied reports that he discussed U.S. sanctions during the call, but later said he "couldn't be certain" what he had talked about.

The text of Flynn's resignation letter is below.

The White House had been warned for weeks, according to the Washington Post, that Flynn was "potentially vulnerable to Russian blackmail." 

In late December, Flynn made a phone call to Ambassador Sergey I. Kislya. He said initially that only scheduling matters were discussed, and then-new sanctions against Russia for election interference were not. He was backed up on that claim by Trump affiliates, including Pence. 

However, the call was intercepted by intelligence officials, and it came out that Flynn had, on the call, attempted to assuage Russian concerns about the sanctions—an unprecedented move for someone who was then a civilian.

There was additional concern about whether money Flynn received from the Russian government during a trip to the country in 2015 violated the Emoluments Clause of the Constitution.

The White House released a statement naming Lt. General Joseph Kellogg as Acting National Security Advisor.