President Trump has blamed James Comey's sacking on his handling of the email scandal involving election-campaign rival Hillary Clinton.
But others are linking it to the director's investigation of Russian contacts with the Trump campaign.
When Donald Trump became United States president, he praised FBI director James Comey for probing Democratic rival Hillary Clinton's email use as secretary of state.
After the inauguration ceremonies in January, Mr Trump warmly greeted him with a very public handshake.
"Oh, and there's James. He's become more famous than me."
By contrast, Hillary Clinton was -- and still is -- blaming Mr Comey for her surprise loss in November's election.
But only months after Mr Trump's high praise, it is the Republican president who has stunned Washington by sacking him.
Mr Trump says the director is no longer able to effectively lead the bureau because of his handling of the email affair.
The Democrats say it has more to do with this dramatic testimony by Mr Comey to Congress in March.
"I have been authorised by the Department of Justice to confirm that the FBI, as part of our counterintelligence mission, is investigating the Russian government's efforts to interfere in the 2016 presidential election. And that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia's efforts. As with any counterintelligence investigation, this will also an include an assessment of whether any crimes were committed."
In his publicly released letter to the ousted director, Donald Trump makes a point of noting Mr Comey had assured him several times he was not personally under investigation.
There are also suggestions the move was prompted by inaccurate testimony Mr Comey gave to a Senate hearing.
White House press secretary Sean Spicer was revealing little when quizzed on the FBI boss's future in a press briefing shortly before the sacking was announced.
(Spicer:) "I mean, I'm aware of the testimony that occurred in the inquiries, but I've yet to follow up on that. I'd be glad to follow up."
(Reporter:) "Does the President still have confidence -- full confidence -- in FBI director James Comey?"
(Spicer:) "I have no reason to believe ... I haven't asked him ... I have not asked the President since the last time we spoke about this."
US Senate Democratic leader Chuck Schumer says the sacking is the latest in a troubling trend by the Trump administration.
"President Trump called me and informed me he was firing Director Comey. I told the President, 'Mr. President, with all due respect, you are making a big mistake.' The first question the administration has to answer is, 'Why now?' If the administration had objections to the way Director Comey handled the Clinton investigation, they had those objections the minute the President got into office. But they didn't fire him then. Why did it happen today? We know the FBI has been looking into whether the Trump campaign colluded with the Russians, a very serious offence. Were these investigations getting too close to home* for the President?"
Republican senator and former presidential candidate John McCain seemed almost lost for words when he was asked about the news.
(McCain:) "Uh, I regret that that took place. The President does have that authority, so I respect that. Okay? Thank you."
(Reporter:) "Thank you very much."
(McCain:) "Thank you."
His clear shock reflected that of other Washington powerbrokers.
Illinois Democratic senator Dick Durbin questioned the future of the FBI's Russia investigation.
"Mr President, the termination and removal of James Comey as director of the Federal Bureau of Investigation raises the critical question as to whether the FBI investigation of Russian interference in the last presidential campaign will continue and whether the investigation into any collusion or involvement by the Trump campaign will also be investigated by the FBI. Any attempt to stop or undermine this FBI investigation would raise grave constitutional issues."
