In his resignation letter, Flynn wrote that he apologized to President Trump and Pence for "inadvertently brief[ing] the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador."
In the weeks before Trump was sworn in as president, Flynn and Russian Ambassador Sergey Kislyak discussed sanctions the same day they were imposed, current and former U.S. officials confirmed to ABC News last week.
In an interview with the Washington Post last week, Flynn denied this. Pence also denied the reports in a January interview, based on information Flynn had given him.
Here's what we know about Flynn's communications with Kislyak and the Trump administration's handling of the situation:
Friday, Nov. 18, 2016Trump names Flynn as his national security adviser.
Sunday, Dec. 25, 2016Flynn and Kislyak exchanged holiday greetings over texts, according to White House press secretary Sean Spicer.
Spicer told reporters in a transition team phone call Jan. 13 that Flynn had texted Kislyak, wishing the Russian ambassador Merry Christmas and Happy New Year. Flynn also said he looked forward to touching base and working with Kislyak, Spicer said.
Thursday, Dec. 29, 2016Firing back at alleged Russian efforts to influence the election, the Obama administration announced it was expelling 35 Russian intelligence operatives from the United States and placing sanctions on five Russia entities.
"I have ordered a number of actions in response to the Russian government's aggressive harassment of U.S. officials and cyber-operations aimed at the U.S. election," Obama wrote in a statement. "These actions follow repeated private and public warnings that we have issued to the Russian government and are a necessary and appropriate response to efforts to harm U.S. interests in violation of established international norms of behavior."
The Russian ambassador sent Flynn a text message asking whether they could talk over the phone. Flynn accepted the invitation and the two spoke by phone that day, according to Spicer.
Flynn and Kislyak's call "centered around the logistics of setting up a call with the president of Russia and the president-elect after he was sworn in," Spicer said, adding, "They exchanged logistical information on how to initiate and schedule that call. That was it. Plain and simple."
Spicer later told ABC News the two discussed a number of topics on the phone, including the crash of a Russian military plane carrying an army choir on Christmas Day and an invitation from the Russian government to the incoming Trump administration to attend upcoming Syrian peace talks.
Spicer stressed to ABC News that Flynn and Kislyak did not discuss the sanctions.
Friday, Jan. 13, 2017In a phone call briefing reporters on the transition period when Trump was president-elect, then-incoming press secretary Sean Spicer provided a tick-tock of Flynn's communications with Kislyak. Spicer detailed Flynn's text messages on Dec. 15 and Flynn's phone call on Dec. 29.
Sunday, Jan. 15, 2017In an interview on CBS' "Face the Nation," Pence said that Flynn told him that conversation centered around "Christmas wishes" and "sympathy for the loss of life in the airplane crash that took place" Dec. 25.
"It was strictly coincidental that they had a conversation," Pence said. "They did not discuss anything having to do with the United States' decision to expel diplomats or impose censure against Russia."
A senior administration official told ABC News earlier this month that Pence's information had come from speaking with Flynn directly.
Saturday, Jan. 21, 2017A follow-up phone call occurred between Flynn and Kislyak to discuss setting up a call between Trump and Russian President Vladimir Putin, Spicer clarified in a press briefing on Jan. 23.
Monday, Jan. 23, 2017During the White House press briefing, Spicer reiterated that the only topics Flynn and Kislyak discussed were holiday greetings, the deadly December plane crash carrying the Russia military choir, the conference in Syria on ISIS and to set up a call between Putin and Trump.
Thursday, Jan. 26, 2017The Justice Department's then-acting Attorney General Sally Yates informed White House counsel Don McGahn that they were misled and expressed concerns that Russia might try to blackmail Flynn. ABC News confirmed through a source close to Yates that U.S. authorities had captured a phone call between Flynn and Russia's ambassador to the United States discussing sanctions. Spicer confirmed the date the DOJ informed the White House in a Feb. 14 White House press briefing.
After he had been informed by the DOJ, McGahn briefed the president and a small group of aides on Flynn, Spicer said at his Feb. 14 press conference. The president asked McGahn to conduct a review to determine whether there was a legal situation. McGahn determined "within several days" there was not a legal issue, Spicer said, without providing further details.
Spicer stressed Flynn's resignation did not derive from a legal issue nor did Flynn do anything "that was a violation of any sort," but that Trump concluded that he no longer trusted his national security adviser.
"The issue here was that the president got to the point that Gen. Flynn's relationship misleading the vice president and others or the possibility that he had forgotten critical details of this important conversation had created a critical mass and unsustainable situation," Spicer said. "That's why the president decided to ask for his resignation and he got it."
Thursday, Feb. 9, 2017The Washington Post reported that Flynn discussed sanctions with Russia's ambassador to the United States before Donald Trump took office. The Post reported that on Feb. 8 Flynn denied twice that he discussed sanctions with Kislyak.
Current and former U.S. officials confirmed to ABC News that Flynn and Kislyak spoke about Russia sanctions, but were unable to say that explicit promises were made to lift the sanctions. Officials said the discussion was under the context that the incoming Trump administration would have a chance to review the sanctions put in place by Obama administration.
Friday, Feb. 10, 2017Flynn's story begins to change. A senior administration official told ABC News that Flynn didn't recall the issue of sanctions ever coming up in his conversations with Kislyak, but "isn't completely certain."
The Kremlin confirmed that Flynn spoke by phone with Kislyak, but said reports that the two discussed sanctions were "wrong."
Flynn traveled to Florida with Trump aboard Air Force One. During the flight, reporters asked Trump about the Washington Post story while on the way to Mar-a-Lago for the weekend. "I don't know about that. I haven't seen it," Trump said. "What report is that?"
He added, "I haven't seen that. I'll look into that."
A White House official later said Trump's "full day" contributed to his lack of knowledge of the story.
Two top Democrats on the House Intelligence Committee -- Reps. Adam Schiff, D-Calif. and Eric Swalwell, D-Calif. -- released statements calling for Flynn to be removed or suspended from his position. The allegations raise "serious questions of legality and fitness for office," Schiff said in a statement. If the allegations are true, Flynn "should no longer serve in this administration or any other," he said. Swalwell said in a statement: "The White House should immediately SUSPEND National Security Advisor Flynn & REVOKE access to classified information until investigated."
Saturday, Feb. 11, 2017Pence spoke twice with Flynn on Friday Feb. 10, which a senior administration official confirmed to ABC News Saturday. Flynn spoke met with Pence Friday morning and then over phone in the evening.
The White House official would not discuss the content of their discussions.
Sunday, Feb. 12, 2017White House policy adviser Stephen Miller faced questions about Flynn as he did several TV interviews Sunday morning.
"I don't have any information to change anything that has previously already been said by the White House on this matter," Miller said. "General Flynn has served this country admirably and with distinction."
When asked by NBC whether the president still has confidence in Flynn, Miller demurred, saying, "That's the question that I think you should ask the president, the question you should ask Reince [Priebus], the chief of staff."
Monday, Feb. 13, 2017House Minority Leader Nancy Pelosi, D-Calif., released a statement calling on Flynn to be "fired immediately." "We have a national security adviser who cannot be trusted not to put Putin before America," the statement read.
A senior White House official told ABC News' Jonathan Karl that Flynn called Pence Feb. 10 to apologize for misleading him about his conversation with the Russian ambassador.
Kellyanne Conway, counselor to the president, told MSNBC that Flynn "enjoy[s] the full confidence of the president." An hour later, however, White House chief of staff Reince Priebus released a statement that said, "the president is evaluating the situation."
Late Monday night, Mike Flynn resigned from his position as national security adviser. The retired lieutenant general released a letter of resignation in which he apologized to Trump and Pence.
"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," Flynn's letter read. "I have sincerely apologized to the President and the Vice President, and they have accepted my apology."
Tuesday, Feb. 14, 2017Counselor to the president Kellyanne Conway said in an interview on "Good Morning America" that she "did not know" whether Trump and Pence were aware three weeks ago that Flynn had misled them about the phone calls initially after the Justice Department relayed its warning about Flynn to the White House counsel.
"I'm not here to say who knew what when because first of all that would be divulging information that is highly sensitive," she said. "And, secondly, I don't know all the details."
Sean Spicer, in a White House press briefing, confirmed that the president asked for Flynn's resignation.
Spicer also said the White House counsel briefed the president on the DOJ's finding the same day the White House counsel was informed.
"The President was informed of this, he asked the White House counsel to review the situation. The first matter was whether there was a legal issue. We had to review that, whether there was a legal issue, which the White House counsel concluded there was not. As I stated in my comments, this was an act of trust. Whether or not he actually misled the Vice President was the issue."
Editor's Note: A previous version of this story incorrectly stated when the DOJ told the White House counsel that Michael Flynn had misled them. It was Jan. 26, not Jan. 23.
ABC News' Pierre Thomas and Jonathan Karl contributed to this report.