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Michael Flynn abruptly quit as President Donald Trump's national security adviser Monday night, hours after it emerged that the Justice Department informed the White House that it believed he could be subject to blackmail.

The resignation also came after previous disclosures that Flynn had misled Vice President Mike Pence and other senior officials about his communications with Sergey Kislyak, Russia's ambassador to the United States. Pence repeated the misinformation in television appearances.

"Unfortunately, because of the fast pace of events, 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 said in his resignation letter.

Michael Flynn waits for an elevator in the lobby of Trump Tower in New York in December. Drew Angerer / Getty Images

Flynn's discussions had raised a possible breach of the Logan Act, a 1799 law that bars unauthorized citizens from negotiating with foreign governments. However, a senior intelligence official last week told NBC News there had been no finding that Flynn did anything illegal.

After "agonizing" for days over the situation, Trump and his top advisers concluded Flynn's position had become unsustainable because he had lied to the president and the vice president, another senior official told NBC News Monday night.

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“That makes no sense.” @MLauer to @KellyannePolls while discussing Trump's former national security adviser Michael Flynn pic.twitter.com/94SaFlQJxo — TODAY (@TODAYshow) February 14, 2017

A senior official also confirmed part of a Washington Post report that Sally Yates, then-acting attorney general, told the White House last month that Flynn was vulnerable to blackmail. Trump fired Yates after she directed Justice Department lawyers not to defend the president's executive order on immigration.

On the TODAY show Tuesday morning, White House counselor Kellyanne Conway said that the president was "very loyal" to Flynn and that the general "decided it was best to resign" because he had "become a lightning rod."

When asked by Matt Lauer would Trump have supported Flynn's decision to not resign despite accusations of him lying to the vice president, Conway responded, "That fact is what became unsustainable, I think misleading the vice president was the key."

Related: Kellyanne Conway on Michael Flynn: 'He knew He Had Become a Lightning Rod'

There was barely-concealed glee in some quarters over the departure of Flynn — an outspoken opponent of political correctness who last year wrote on Twitter that "fear of Muslims is rational."

Republican Rep. Bill Flores of Texas said he was "glad" Flynn had gone, adding: "We need more sanctions on Russia, not fewer!"

Rep. Adam Schiff of California, the ranking Democrat on the House Intelligence Committee — which is investigating the Trump campaign's alleged contacts with Russia — said Flynn's resignation was "all but ordained the day he misled the country about his secret talks with the Russian ambassador."

"In fact, Flynn was always a poor choice for National Security Adviser, a role in which you need to be a consensus builder, and possess sobriety and steady judgment," Schiff said in a statement. "It is certainly no role for someone who plays fast and loose with the truth."

However, Republican Rep. Devin Nunes of California, the chairman of the House Permanent Select Committee on Intelligence, thanked Flynn for his "many years of distinguished service."

Related: Flynn in Hot Seat Over Discussing Sanctions With Russians

"Michael Flynn served in the U.S. military for more than three decades. Washington, D.C. can be a rough town for honorable people, and Flynn — who has always been a soldier, not a politician — deserves America's gratitude and respect for dedicating so much of his life to strengthening our national security," Nunes said in a statement.

Retired Army Gen. Keith Kellogg, a top policy adviser for Trump's campaign, was appointed acting national security adviser, the White House said.

Kellogg, 72, a former commander of the fabled 82nd Airborne Division, was chief operating officer of the Western coalition in Baghdad, Iraq, after the U.S.-led invasion in 2003.

Also under consideration for the permanent role is retired Navy Vice Adm. Robert Harward, former deputy commander of U.S. Joint Forces Command, and former CIA Director David Petraeus. Three senior U.S. officials told NBC News that Harward was considered the favorite for the job.