His departure came just after reports surfaced that the Justice Department warned the Trump administration
last month that Flynn misled administration officials regarding his communications with the Russian ambassador to the United States and was potentially vulnerable to blackmail by the Russians.
"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," Flynn wrote, according to a copy of his resignation letter
obtained by CNN.
"I am tendering my resignation, honored to have served our nation and the American people in such a distinguished way," he wrote. "I know with the strong leadership of President Donald J. Trump and Vice President Mike Pence and the superb team they are assembling, this team will go down in history as one of the greatest presidencies in US history."
The move comes less than a month into the job, making him one of the shortest-serving senior presidential advisers in modern history.
Gen. Keith Kellogg will be the interim national security adviser, multiple sources tell CNN. He most recently served as National Security Council chief of staff.
A senior administration official said Kellogg, retired Gen. David Petraeus and former Vice Admiral Bob Harward
are possible replacements for Flynn. Another senior official told CNN Tuesday that Harward is considered the top contender for the job.
Petraeus is going to the White House Tuesday, according to sources inside and close to the administration.
"He is making a run" for the job, one source said, but noted "he has a lot of baggage."
The sudden exit marks the most public display yet of disarray at the highest levels of the new administration, which has faced repeated questions over a slew of controversies and reports of infighting among senior aides during its first three weeks.
The resolution had been heading this way for three days, an administration source told CNN.
More than whether he really had a conversation with the Russians about sanctions, the key issue internally was whether he told the truth to Pence, the source said.
The White House concluded at the very least, Flynn didn't mean to mislead the vice president, but may have because he couldn't remember what he said to the Russians.
"Not remembering is not a quality we can have for the national security adviser," the source said.
An administration source said that Trump "hung in there" when it came to Flynn, but there was a "flood of information" that finally made it clear he had to resign.
Asked if Trump is disappointed, another administration official said: "He's moving on."
'Full classified briefing'
A pair of Democratic lawmakers -- Reps. John Conyers, Jr., top Democrat on the House Judiciary Committee, and Elijah Cummings, top Democrat on the House oversight Committee -- sent a request for a "full classified briefing" on the circumstances surrounding Flynn to the Justice Department and FBI Monday night following Flynn's resignation.
"We in Congress need to know who authorized his actions, permitted them and continued to let him have access to our most sensitive national security information despite knowing these risks. We need to know who else within the White House is a current and ongoing risk to our national security," they wrote in a statement.
They added: "This new disclosure warrants a full classified briefing by all relevant agencies, including the Department of Justice and the FBI, as soon as possible and certainly before Thursday, February 16. We are communicating this request to the Department of Justice and FBI this evening."
The shakeup now leaves Trump without one of his closest and longest-serving advisers. Flynn had counseled Trump on foreign policy and national security matters since early in the 2016 presidential race.
Flynn was not able to definitively refute a Washington Post story late last week that his conversations with Russian Ambassador Sergey Kislyak included communication about the sanctions. It is illegal for unauthorized private citizens to negotiate with foreign governments on behalf of the US.
The controversy intensified
after the report put Vice President Mike Pence and several senior White House advisers in an uncomfortable position, as they had denied in TV interviews weeks earlier that Flynn discussed sanctions with the ambassador. Some administration officials said Flynn must have misled Pence and others.
"The knives are out," a White House official told CNN on Friday, noting that "there's a lot of unhappiness about this."
Many expressed concern at the idea that Flynn, a retired lieutenant general who headed the Defense Intelligence Agency, would discuss sanctions with a foreign official whose calls are regularly monitored by US intelligence and law enforcement agencies.
A US official confirmed to CNN on Friday that Flynn and Kislyak did speak about sanctions, among other matters, during a December call.
But after the call was made public, Pence told CBS News on January 15 that Flynn did not talk with Kislyak about the sanctions, which the Obama administration recently levied due to Russia's alleged interference in the 2016 elections.
"They did not discuss anything having to do with the United States' decision to expel diplomats or impose censure against Russia," Pence told CBS News.
On Friday, an aide close to the national security adviser told CNN that Flynn could not rule out that he spoke about sanctions on the call.
The White House official blamed much of the outcry against Flynn on a Washington culture always in search of a scalp, but people within Trump's orbit did little to defend Flynn during appearances on Sunday news shows.
Stephen Miller, White House policy director, was asked directly about Flynn's future on a number of Sunday talk shows. Miller responded by saying he was not the appropriate official to ask.
"I don't have any answers today," Miller said in response to questions about whether Flynn misled the vice president. "I don't have any information one way or another to add anything to the conversation."