Michael Flynn's sudden downfall and resignation from his post as national security adviser to U.S. President Donald Trump has spurred calls for a deeper investigation into the new administration's ties with Russia.

His exit Monday lit a political firestorm and prompted questions about the integrity of U.S. national security and more simply who knew what and when. Here's a closer look at some of the key players in the Flynn affair.

flynn

Michael Flynn resigned as U.S. national security adviser late Monday evening, amid allegations that he made improper contact with Russia and misled the vice-president. White House spokesman Sean Spicer said Trump called for him to resign, saying the president's trust in Flynn had eroded over the past several weeks.

Flynn spoke by phone with Russian Ambassador Sergey Kislyak last December after the U.S. announced a series of sanctions on Russia.

While Flynn has acknowledged speaking with Kislyak, he denied the pair discussed foreign policy matters. Such a move would have been a violation of the Logan Act, which bans private citizens from engaging in foreign policy on behalf of the U.S. administration. Flynn didn't officially become national security adviser until Jan. 20 when Trump was sworn in.

But Flynn's narrative has changed. He told the Washington Post on Feb. 8 that he didn't discuss sanctions. A day later, his spokesperson said Flynn wasn't certain the matter hadn't been broached. Shortly before Flynn resigned, he reasserted that his contact was not improper.

"It wasn't about sanctions. It was about the 35 guys who were thrown out," Flynn told the Daily Caller on Monday evening. "It was basically: 'Look, I know this happened. We'll review everything.' I never said anything such as, 'We're going to review sanctions,' or anything like that."

CNN has also suggested U.S. intelligence observed frequent contact between Flynn and senior Russian officials during the campaign. Flynn has not commented on the report. 

Kisylak

Sergey Kislyak is a longtime Russian diplomat and has served as his country's ambassador to the U.S. since 2008.

The Kremlin has been tightlipped on the nature of the conversations between Flynn and Kislyak.

Putin's spokesman, Dmitry Peskov, declined to comment on the matter.

The Washington Post, however, cited nine anonymous sources who said that Flynn and Kislyak did indeed discuss sanctions in December.

Pence

Vice-President Mike Pence became deeply frustrated with Flynn as the narrative unraveled, news agencies have suggested.

On Jan. 15, Pence appeared on CBS's Face the Nation and said it was coincidental that Flynn's conversation with Kislyak took place around the same time the sanctions against Russia were announced.

The vice-president said Flynn sent an innocuous text offering Christmas tidings and expressing sympathy for a Russian military plane crash.

"It was strictly coincidental that they had a conversation," he said. "They did not discuss anything having to do with the United States' decision to expel diplomats or impose censure against Russia."

Yates

Former acting attorney general Sally Yates and an unnamed national security official alerted the White House in January that there were inconsistencies in Flynn's account.

The White House is then said to have examined a transcript of a conversation between Flynn and Kislyak, according to the New York Times. Intelligence gathering officials routinely record calls that foreign officials make while in the U.S.

Yates warned the White House that the national security adviser was vulnerable to Russian blackmail, according to the Washington Post.

Yates, who was fired by Trump on Jan. 30 after she refused to defend his controversial travel ban, reportedly first approached FBI Director James Comey with her concerns and later Don McGahn, White House counsel.

Manafort

Paul Manafort, Trump's former campaign manager, is again embroiled in controversy after reports suggested Trump's top aides were frequently in contact with senior Russian officials during the campaign.

Manafort denies the claims though the reports have raised questions about the ties between the White House and the Kremlin.

"I don't remember talking to any Russian officials, ever," Manafort told CNN in a report published on Feb. 15. "Certainly during the time we're talking about."

Manafort resigned from the campaign in August amid reports that he had pro-Russian lobbying interests.

trump

U.S. President Donald Trump has tweeted about the scandal but has placed emphasis on the leaks, rather than Flynn.

He also accused news agencies including CNN and MSNBC of putting forth "conspiracy stories" and "fake news."

At a news conference on Tuesday, Sean Spicer, White House press secretary, also dismissed suggestions that the White House was too cozy with Russia.

"The irony of all this has been the president has been incredibly tough on Russia," Spicer said.

With files from The Associated Press