Michael Flynn, the national security adviser to President Trump, resigned late Monday over revelations about his potentially illegal contacts with the Russian ambassador to the United States, and his misleading statements about the matter to senior Trump administration officials.

Flynn stepped down amid mounting pressure on the Trump administration to account for its false statements about Flynn’s conduct after The Washington Post reported Monday that the Justice Department had warned the White House last month that Flynn had so mischaracterized his communications with the Russian diplomat that he might be vulnerable to blackmail by Moscow.

[Justice Department warned White House that Flynn could be vulnerable to Russian blackmail, officials say]

In his resignation letter, Flynn said he had “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.”

Flynn was referring to his disproven claims to Vice President Pence and others a month ago that he had never discussed U.S. sanctions against Moscow with Russian Ambassador Sergey Kislyak. Pence, White House spokesman Sean Spicer and others, relying on Flynn’s accounts, publicly defended him and repeatedly declared in categorical terms that sanctions were never discussed.

(Sarah Parnass/The Washington Post)

President Trump accepted Flynn’s resignation letter and appointed Keith Kellogg, a decorated retired Army lieutenant general, as acting national security adviser.

Flynn’s resignation — after just 24 days on the job — caps a decorated career in public service for the retired lieutenant general and intelligence official.

Kellogg is one of three candidates Trump is considering as a permanent replacement for Flynn, according to a senior White House official. The other two are David H. Petraeus, a former CIA director and retired general, and Vice Adm. Robert Harward, a former deputy commander of the U.S. Central Command.

One senior White House official said that Trump did not fire Flynn; rather, Flynn made the decision to resign on his own late Monday evening because of what this official said was “the cumulative effect” of damaging news coverage about his conversations with the Russian envoy.

[The imeline of resignation is troubling for the White House]

This official, who requested anonymity to speak candidly about the situation, said Trump does not relish firing people — despite his television persona on “The Apprentice” — and had intended to wait several more days before deciding whether to seek Flynn’s resignation.

“There obviously were a lot of issues, but the president was hanging in there,” this official said. “Buying some time was part of the plan, and I think Flynn just figured, if it’s imminent to the boss, then let’s make it immediate.”

(Deirdra O'Regan/The Washington Post)

Flynn’s departure just weeks into the Trump administration compounds the confusion in the National Security Council that is supposed to serve as a disciplined coordination center for the administration’s handling of international affairs.

Instead, the White House faces an escalating court fight over an immigration ban aimed at Muslim-majority countries, has alienated key allies with Trump’s brusque phone calls to foreign leaders, and seemed so caught off-guard by North Korea’s recent ballistic missile test that Trump and senior officials were shown learning of the development on cellphones in full view of patrons at Trump’s Mar-a Lago resort.

Flynn was forced out less than a week after it was disclosed that he had discussed U.S. sanctions against Russia with that country’s ambassador before Trump was sworn in as president.

But Flynn’s undoing was more directly tied to his inaccurate accounts of those contacts to senior Trump officials including Pence, who officials said was incensed to learn that Flynn has not told him the truth.

[The fall of Michael Flynn: A timeline]

Flynn again denied that he had discussed the subject in an interview with The Washington Post last week, only to back away from that statement a day later by acknowledging, through a spokesman, that while he couldn’t recall speaking about sanctions he could not rule it out.

In fact, U.S. intelligence and law enforcement officials have said that sanctions were a main subject of Flynn’s conversation with Kislyak on the day that the Obama administration announced a series of punitive measures aimed at punishing Moscow for its meddling in the 2016 U.S. presidential election.

U.S. officials said that Flynn told Kislyak that Moscow should not overreact to the sanctions, indicating that the two sides would soon be in position to revisit the matter, presumably in Moscow’s favor.

In conveying that message, Flynn may have broken a law against unauthorized individuals negotiating with foreign governments over conflicts. He is unlikely to face legal sanction, however, because that law dates to 1799 and has never been used in a prosecution.

But Flynn’s departure is unlikely to end the trouble the issue has created for the Trump administration. The Post reported Monday that then-acting U.S. attorney general Sally Q. Yates told the White House counsel last month that Flynn’s misleading statements to Pence and others made him vulnerable to blackmail by Russia, whose own government would have known that sanctions were discussed.

The White House appears to have let its repeated false statements about Flynn stand for weeks after that notification from Yates, and has yet to account for what it did with the warning she conveyed. The disclosures about Flynn have added to the swirling suspicion about the Trump administration’s relationship with Moscow — suspicion based in part on Trump’s repeated expressions of admiration for Russian President Vladi­mir Putin.

Flynn’s resignation appears to end the career of a highly decorated U.S. military intelligence officer, who served repeated tours in Afghanistan and Iraq but became a polarizing figure in last year’s presidential campaign.

In a speech at the Republican National Convention, Flynn led vitriolic attacks on Democratic nominee Hillary Clinton, leading chants of “Lock her up” and declaring that if he had been even partly as careless as she was in her handling of sensitive material by email, he would be in jail.

[He was one of the most respected intel officers of his generation. Now he’s leading ‘Lock her up’ chants.]

Flynn spent last weekend at Mar-a-Lago with Trump, staffing the president during his visit with Japanese Prime Minister Shinzo Abe. Back at the White House on Monday, Flynn attended classified briefings, helped orchestrate the visit of Canadian Prime Minister Justin Trudeau, and led Trump’s morning intelligence briefing, this official said.

Some of Trump’s political advisers felt Flynn should not be fired in the midst of intense media scrutiny and calls for his resignation from Democratic opponents.

“Part of me said, nobody should be firing this guy — not on the day that Nancy Pelosi said fire this guy,” the official said. “You’ve got hashtag ‘Fire Flynn’ blazing across the Internet by a bunch of Trump detractors.”

But by Monday evening, Flynn had decided he could not survive.

“It was when you feel like you’re looking around the room and asking, ‘Where’s my friend?’” the White House official said. “The Pence thing was huge. He is not somebody who’s quick to anger. That was very telling to everybody.”

Flynn presented his resignation letter to Trump roughly around 9 p.m., shortly after Treasury Secretary Steven Mnuchin’s swearing-in ceremony in the Oval Office. Trump accepted the letter.

“It was a sad moment,” the White House official said.

Read more:

Harward emerges as front-runner to replace Flynn following talks

Russian lawmakers rush to the defense of Trump’s ex-national security adviser

10 unanswered questions after Flynn’s resignation