The bombshell announcement has surprised US politicians on both sides of the political divide...
Donald Trump's presidency to date has been full of drama, revelations and controversies - but even by the standards of an extraordinary administration, the firing of FBI director James Comey was a bombshell development.
"Many hear echoes of Watergate," declared one New York Times headline. The Washington Post announced: "The immediate echo: 'Saturday Night Massacre'" (referring to Richard Nixon's order to fire a special prosecutor in the midst of the Watergate Scandal).
Washington veterans reacted with ill-disguised surprise to Tuesday's announcement, with some Republicans joining Democrats in voicing concern over President Trump's decision.
Many may have expected Democrats - who heavily criticised Comey for his handling of the Hillary Clinton email investigation, especially in the aftermath of his dramatic intervention just days ahead of the US election - to welcome Mr Comey's dismissal.
But it is the spectre of the FBI's ongoing investigation into alleged Russian interference in the election campaign - and potential links between Russia and the Trump campaign - that has made the firing such an instant controversy...
James Comey - a former US deputy attorney general - was appointed by Barack Obama to lead the Federal Bureau of Investigation in 2013, and subsequently confirmed by the Senate.
Like all FBI directors, Comey was appointed for a ten-year term - a limit introduced after J Edgar Hoover's decades-long term.
Welcoming Mr Comey, Obama said: "It’s just about impossible to find a matter of justice he has not tackled, and it’s hard to imagine somebody who is not more uniquely qualified to lead a bureau that covers all of it".
While he did not entirely avoid controversy during his first years in office, Mr Comey was propelled to international prominence as a result of his agency's investigation into the use of Hillary Clinton's use of a private email server while she was US secretary of State.
The investigation was concluded in July 2016 - only weeks before Mrs Clinton was confirmed as the party's presidential candidate at the Democratic National Convention - when Mr Comey held a press conference announcing their findings.
The FBI director himself acknowledged it was an 'unusual statement', saying: "I am going to include more detail about our process than I ordinarily would, because I think the American people deserve those details in a case of intense public interest".
He stressed that he had not reviewed the statement with Obama's justice department, and emphasised that agents operated in 'an entirely apolitical' way.
He explained: "Although we did not find clear evidence that Secretary Clinton or her colleagues intended to violate laws governing the handling of classified information, there is evidence that they were extremely careless in their handling of very sensitive, highly classified information."
However, he also noted: "Although the Department of Justice makes final decisions on matters like this, we are expressing to Justice our view that no charges are appropriate in this case."
Although Mr Comey faced criticism for his handling of the 'unusual' announcement, it did appear to bring the official investigation into the emails to an end - even if 'the emails' remained a major source of rhetorical ammunition for Mrs Clinton's opponents, including Donald Trump.
Comey and the email scandal made headlines again in late October of last year, with less than two weeks to go until the election.
In a bombshell letter to members of Congress, Mr Comey indicated that his organisation was investigating newly discovered emails connected to Mrs Clinton's use of the private server. The emails were found on a laptop belonging to former congressman Anthony Weiner, the husband of senior Clinton aide Huma Abedin.
Speaking to supporters after the bombshell letter, Donald Trump praised the FBI's director's actions, saying: "It took guts for director Comey to make the move that he made [...] I really disagreed with him, I was not his fan. What he did, he brought back his reputation."
The move did, however, draw criticism from Democrats and some Republicans alike.
Only days before the election, Comey announced no evidence of wrongdoing had been found after the emails were analysed. But the 'damage' had been done.
Mrs Clinton herself has claimed the letter was one of the factors that led to her electoral defeat - a stance supported by prominent pollster and analyst Nate Silver.
Last week, Mr Comey said: "It makes me mildly nauseous to think we might have had some impact on the election, but honestly it wouldn't change the decision."
Ostensibly, Comey's (mis)handling of the email investigation has been cited by the Trump administration as the reason behind the dismissal of Mr Comey. Indeed, the issue has continued to attract controversy: only this week US media outlet Pro Publica reported that Mr Comey’s Senate testimony on Abedin forwarding emails to her husband was inaccurate.
In a letter to Trump, US Deputy Attorney Rod Rosenstein argued: "I cannot defend the Director's handling of the conclusion of the investigation of Secretary Clinton's emails, and I do not understand his refusal to accept the nearly universal judgement that he was mistaken.
"Almost everyone agrees that the Director made serious mistakes; it is one of the few issues that unites people of diverse perspectives."
Yet it is another investigation entirely that has led to such unease in Washington over the shock dismissal.
Ever since the election, there is one controversy the young administration has been unable to shake: Russia.
There have been widespread allegations about Russia's supposed influence in the US election campaign, ranging from a campaign of 'strategic messaging' in Russian state media to 'cyber operations' against US political parties.
A US intelligence community report in CIA concluded: "We assess Russian President Vladimir Putin ordered an influence campaign in 2016 aimed at the US presidential election. Russia’s goals were to undermine public faith in the US democratic process, denigrate Secretary Clinton, and harm her electability and potential presidency. We further assess Putin and the Russian Government developed a clear preference for President-elect Trump."
While much of the controversy involves allegations and speculation, it has also manifested itself very publicly: in February, for example, national security adviser Michael Flynn resigned after claims he misled the Trump administration over his talks with Russia.
In March, Mr Comey confirmed the FBI is investigating any possible links between members of the Trump campaign and the Russian government.
"That includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government; and whether there was any co-ordination between the campaign and Russia's efforts," he said.
It is that investigation that has led to such outrage over Mr Comey's dismissal.
Democratic leader Nancy Pelosi observed: “The President’s sudden and brazen firing of the FBI Director raises the ghosts of some of the worst Executive Branch abuses.
"We cannot stand by and watch a coverup of the possible collusion with a hostile foreign power to undermine American democracy."
Most Republicans backed the president, but there was some unease.
Veteran Republican Senator John McCain said that while a president can legally remove an FBI director, he was 'disappointed' at President Trump's decision.
He argued: "I have long called for a special congressional committee to investigate Russia's interference in the 2016 election. The president's decision to remove the FBI Director only confirms the need and the urgency of such a committee."
As calls for a special committee or prosecutor for the Russia investigation mount, the Trump administration has moved to defend the choice to dismiss Comey.
Trump himself tweeted: "The Democrats have said some of the worst things about James Comey, including the fact that he should be fired, but now they play so sad!
"Comey lost the confidence of almost everyone in Washington, Republican and Democrat alike. When things calm down, they will be thanking me!"
James Comey will be replaced by someone who will do a far better job, bringing back the spirit and prestige of the FBI.— Donald J. Trump (@realDonaldTrump) May 10, 2017
Whatever happens, it's clear that the dismissal was an audacious and risky political gambit by Donald Trump, even accepting the official White House justification for the move.
It is likely to refocus attention on the Russia investigation, and politicians will be keeping a close eye on President Trump's choice to succeed Mr Comey for any whiff of partisan bias (deputy director Andrew G McCabe is serving as acting director for now, and a permanent successor will require Senate approval).
Donald Trump has proven himself to be an unpredictable leader who often courts controversy - and the Comey episode is already proving one of most remarkable chapters in the presidency to date.