Donald Trump's embattled national security adviser Michael Flynn resigned las than month after taking office, following reports that he had misled Vice President Mike Pence and other officials about his contacts with Russia.
Mr Flynn had been embroiled in controversy since it was revealed that he had a conversation with a Russian diplomat about sanctions prior to the US President's administration taking office.
In a resignation letter, Mr Flynn said he gave Vice President Mike Pence and others "incomplete information" about his calls with Russia's ambassador to the US.
Apparently relying on this information, Mr Pence initially said the national security adviser had not discussed sanctions with the Russian envoy, though Mr Flynn later conceded the issue may have come up.
His resignation letter said that he held numerous calls with "foreign counterparts, ministers, and ambassadors ... to facilitate a smooth transition".
He added: "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 apologised to the President and the Vice President, and they have accepted my apology."
"I am tendering my resignation," Mr Flynn wrote, "honoured to have served our nation and the American people in such a distinguished way."
The White House said it will replace Mr Flynn with retired Army Lieutenant General Joseph K Kellogg Jr as acting national security adviser.
Officials who had seen the transcripts of Mr Flynn's conversation with the Russian Ambassador, said he had touched on the subject of the sanctions imposed after the intelligence community determined the Russian government was behind the 2016 hacks of the Democratic National Committee – an alleged attempt to sway the election in Mr Trump's favour.
It is considered a drastic break in protocol to discuss policy with foreign officials, let alone those in an adversarial government, before the new administration takes office.
Mr Flynn had originally denied that he had the conversations, at one point claiming that he could not remember what was discussed in his talks with the Russian diplomat.
When first asked about the situation, Mr Trump was aboard Air Force One with Japanese Prime Minister Shinzo Abe. He claimed to have no idea what reporters were referring to.
"I don't know about that, I haven't seen it," Mr Trump said. "What report is that? I haven't seen that. I'll look into that."
The White House later said it was "evaluating the situation" regarding Mr Flynn.
California congressman Adam Schiff, a ranking member of the House Permanent Select Committee on Intelligence, said that questions over Mr Flynn's contact with Russia still remain despite his departure.
"General Flynn's decision to step down as National Security Adviser was all but ordained the day he misled the country about his secret talks with the Russian Ambassador," Mr Schiff said in a statement. "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 judgement.
"It is certainly no role for someone who plays fast and loose with the truth."
House Intelligence Committee chair Devin Nunes struck a different tone in his statement about Mr Flynn, whom he honour for his three decades of service to the US.
"Washington, DC, can be a rough town for honourable people," he said, "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."
Nevertheless, Mr Flynn raised red flags for the Justice Department late last year.
In the final days of the Obama administration, Acting Attorney General Sally Yates – whom Mr Trump fired after she spoke out against his travel ban – had reportedly warned incoming Trump administration officials that "Flynn had put himself in a compromising position".
The Trump administration has been plagued with leaks during its first few weeks, but none had appeared to have the same damaging effect as those related to Mr Flynn.
"This is very new, because usually the administration is united and unified and quiet," Robert Shapiro, a Columbia University government professor, told The Independent.
He added that the leaks were especially damaging to the infant administration because senior officials – including Vice President Mike Pence – had backed Mr Flynn.
“I think Mr Flynn has to go,” he said. “This is an embarrassing distraction.”
Mr Flynn was one of Donald Trump's earliest supporters. During his appearance at the Republican National Convention, the former general led the audience in cheers of "Lock her up!" — a popular rallying cry of Trump supporters who shared the unsubstantiated belief that Hillary Clinton was guilty of criminal misconduct.
"If I did a tenth of what she did," Mr Flynn told the crowd, "I'd be in jail today."