WASHINGTON — A year ago today, President Donald Trump’s newly sworn–in national security adviser, Michael Flynn, met privately in his West Wing office with FBI investigators interested in his communications with Russia's ambassador, without a lawyer or the knowledge of the president and other top White House officials, according to people familiar with the matter.

Flynn's FBI interview on Jan. 24, 2017, set in motion an extraordinary sequence of events unparalleled for the first year of a U.S. presidency. Flynn was fired as national security adviser after 24 days on the job, the acting attorney general was fired 10 days after the president took office, the FBI director was allegedly pressured by the president to let go of an investigation into Flynn, and then eventually fired himself.

The attorney general recused himself from a federal investigation into Russia's meddling in the 2016 U.S. election and possible collusion with the sitting president's campaign, and a special counsel was appointed.

The developments ensnared the president in an obstruction of justice inquiry, which resulted in his top intelligence and law enforcement chiefs cooperating in some form with that probe.

By the end of 2017, special counsel Robert Mueller’s team had spoken with Director of National Intelligence Dan Coats; Mike Rogers, the director of the National Security Agency; former FBI Director James Comey; and numerous members of Trump’s campaign and White House inner circle. Flynn pleaded guilty last month to lying to the FBI during his Jan. 24 interview and is cooperating with the Russia investigation.

NBC News also has learned that former acting Attorney General Sally Yates, who informed the White House about Flynn’s interview two days after it took place, has cooperated with the special counsel. CIA Director Mike Pompeo has also been interviewed, according to people familiar with the inquiry. Pompeo was present at a meeting where Trump allegedly asked for help in leaning on Comey to drop his investigation.

One person familiar with the matter described Pompeo, Coats and Rogers as "peripheral witnesses" to the Comey firing. Attorney General Jeff Sessions, who played a key role in Comey's departure and was a top adviser on the Trump campaign, was interviewed by Mueller last week as the special counsel's team inches closer to possibly questioning the president himself.

Trump's former chief strategist Steve Bannon is expected to meet with Mueller’s team by Jan. 31, according to a person familiar with the discussions.

Unresolved is whether Trump will voluntarily agree to be interviewed by Mueller. So far, according to two people familiar with the discussions, Trump’s team has not reached an agreement with the special counsel for their client to meet with him.

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Flynn's tenure in the White House and his firing by Trump are among the topics Mueller’s team has been asking witnesses to discuss, and they are likely to be part of any questioning of the president. Separately, Mueller is expected to want to ask Trump about his firing of Comey. The president told NBC News last year that his decision was connected to Comey’s dogged pursuit of potential ties between the Trump campaign and Russia. Trump has repeatedly called the investigation a "hoax" and a "witch hunt."

Two people familiar with the matter said Trump was unaware that Flynn had spoken with the FBI until two days after the interview took place. An attorney for Flynn did not respond to a request for comment on this story.

A brief phone call from the office of Andrew McCabe, the deputy FBI director, to a scheduler for Flynn on Jan. 24 set the interview in motion, according to people familiar with the matter. The scheduler was told the FBI wanted to speak with Flynn later that day, these people said, and the meeting was placed on Flynn’s schedule. The scheduler didn't ask the reason for the meeting, and the FBI didn't volunteer it, one person familiar with the matter said.

Later that day, two FBI agents arrived at the White House to speak with Flynn. A lawyer for the National Security Council typically would be informed of such a meeting and be present for it, one person familiar with the procedures said. But that didn't happen in this instance, and Flynn didn't include his own personal lawyer, two people said. He met with the two federal agents alone, according to these people.

"No one knew that any of this was happening," said another senior White House official who was there at the time.

"Apparently it was not clear to Flynn that this was about his personal conduct," another White House official said. "So he didn't think of bringing his own lawyer."

White House counsel Don McGahn was the first senior official to learn of Flynn's interview during a meeting on Jan. 26 with Yates in which she warned him that Flynn had lied to Vice President Mike Pence and other top Trump officials about his conversation with the Russian ambassador, Sergey Kislyak, and could be vulnerable to blackmail by the Russian government.

McGahn has sat for two days of interviews with Mueller's team, according to a person familiar with the matter, including one interview that was rescheduled after Flynn’s plea deal was announced the day it was supposed to take place. Yates spoke extensively with Mueller’s team last year, according to people familiar with the matter.

McGahn briefed Trump, Bannon and White House chief of staff Reince Priebus, who left the White House last summer and also has spoken with Mueller's team, on his meeting with Yates that same day including the news of Flynn's FBI interview, people familiar with the matter said. Yates has testified before Congress that McGahn asked her how Flynn did in his FBI interview, to which she said she replied that she could not comment on it.

McGahn did not later ask Flynn if he lied to the FBI, one person familiar with the matter said. This person said it was unclear if Flynn intended to lie and that McGahn did not conclude that Flynn had lied to the FBI until after he had been fired. It was at that time in late winter or early spring that the White House received a request from the FBI for phone records and other documents related to Flynn that McGahn and other top officials concluded he had lied in his interview and was otherwise under investigation, this person said.

The year since Flynn's FBI interview has seen only escalated tensions between Trump and his Justice Department.

In recent weeks Trump has taken aim at McCabe, whose office first arranged Flynn’s FBI interview.

This week, White House spokesman Raj Shah fanned reports of pressure from the White House to fire McCabe by saying in a statement that Trump "believes politically motivated senior leaders" of the FBI "have tainted the agency's reputation for unbiased pursuit of justice" and that the new director he "appointed" will "clean up the misconduct at the highest levels of the FBI."

And one of the two FBI agents who interviewed Flynn was Peter Strzok, whom Mueller removed from the Russia investigation last summer after the Justice Department’s inspector general's office found he'd written text messages to a colleague criticizing Trump, according to people familiar with the matter.

For Trump opponents, his war with the FBI is an effort to undermine the Russia investigation. For Trump and his allies, he’s battling a conspiracy within the top ranks of the Justice Department to undermine his presidency.

Flynn was fired as Trump’s national security adviser on Feb. 13, after it became public that he had lied to Pence about his conversation with Kislyak.

The next day Trump asked Comey to drop the investigation into Flynn, according to Comey’s testimony before Congress. Trump has denied asking Comey to let the Flynn investigation go.