Donald Trump needs to find someone to head the FBI after unceremoniously sacking James Comey, who the US President said was "not able to effectively lead the bureau".

The move has been criticised by members on both sides of politics, given Mr Comey was partway through an investigation into Russia's election meddling and the possible involvement of Trump associates.

For that reason, Democrats have called for a special prosecutor to be brought in.

Here's what's going on.

Who's running the FBI now?

The acting FBI director is Andrew McCabe, who had been Mr Comey's deputy.

But Mr Trump is expected to appoint an interim replacement, ahead of a permanent one.

FBI directors serve 10-year terms — unless they're sacked, obviously. (Mr Comey was nominated by former president Barack Obama in 2013.)

The Reuters news agency reports there are four candidates in the running for the interim role, including keeping on Mr McCabe in the position.

The others are high-ranking officials within the bureau — assistant FBI director in charge Paul Abbate, Chicago special agent in charge Michael J Anderson and Richmond, Virginia, special agent in charge Adam Lee.

Who's in the running to replace Comey?

Mr Trump is likely to look outside the bureau to find someone to run the law enforcement agency.

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Of course, like cabinet positions, the person nominated by Mr Trump must be confirmed by the Senate.

And while the Republicans have a majority in the Senate to allow the President's pick to be easily passed through, any candidate deemed by the Democrats to be too closely aligned to Mr Trump will likely face heavy scrutiny.

Until then, possible candidates being floated by US media include New Jersey Governor Chris Christie, former New York mayor Rudy Giuliani, former New York police commissioner Ray Kelly, and John Pistole, a former deputy director of the FBI under president George W Bush.

Back up a sec, why was Comey fired?

It was simple, according to Mr Trump: "He wasn't doing a good job. Very simply. He was not doing a good job."

Democrats aren't buying it, arguing Mr Comey's fate is more likely linked to the Russian investigation.

The White House says that's not true and it's encouraging the FBI to complete its investigation.

Kelly Jane Torrance, deputy managing editor at conservative US publication The Weekly Standard, said the timing of Mr Comey's sacking certainly hadn't helped the situation.

"[Mr Trump] might have been justified in doing it if he had done it as soon as he took office … the timing is very important," she said.

"If his administration was concerned about the way that James Comey handled the Hillary Clinton email investigation, they had plenty of time to look into that and examine that in the months leading up to the inauguration and the first days of the administration itself.

"[Now] we find, just days after Comey asked for more resources for the Russia investigation, that Trump suddenly decides, 'Hey, I don't like this guy, he is not doing a good job'.

"Timing is everything. It is why everybody in Washington right now is so abuzz with all this."

What will happen to the Russia investigation?

As mentioned, the White House says the FBI should complete the probe.

Before he was fired, Mr Comey had requested more resources to pursue the investigation, US officials said.

It was unclear whether word of the request, put to deputy Attorney-General Rod Rosenstein, ever made its way to the President.

(You'll remember Mr Rosenstein as the person who wrote the memo criticising Mr Comey's work, which Mr Trump cited as a reason behind the dismissal.)

Democrats want a special prosecutor appointed to investigate Russian involvement in the Trump campaign.

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What's a special prosecutor?

Protesters outside the White House following the sacking of James Comey. ( Reuters: Jonathan Ernst )

An independent lawyer who is brought in when an investigation through regular channels presents a possible conflict of interest.

So in this instance, Democrats question the impartiality of the Republican-led committees in the House and Senate that are also investigating Russian interference during the election.

As they would any future FBI director with political, personal or business links to the President.

A special prosecutor was appointed in 1973 to investigate the Watergate scandal that engulfed president Richard Nixon.

Archibald Cox was supposed to investigate a break-in at the offices of the Democratic National Committee in the Watergate Hotel in Washington DC.

But he had barely started when he was sacked by Mr Nixon. That set in motion a series of events that led to Mr Nixon's resignation in 1974.

The White House thinks the Dems are hyperventilating

White House spokeswoman Sarah Huckabee-Sanders said Democrats condemning Mr Comey's sacking were being hypocritical.

"If Hillary Clinton had won the election, which thank God she didn't, but if she had and she had been in the same position, she would have fired Comey immediately," she said.

"The very Democrats that are criticising the President today would be dancing in the streets celebrating."

Republican House Speaker Paul Ryan also defended the move, saying the President had lost patience with Mr Comey, and many Republicans, Democrats and senior Justice Department officials felt the same way.

Speaking to Fox News, he also said appointing a special prosecutor to investigate Russian ties to Trump's campaign would be a bad idea.

But there are Republicans who've voiced concerns

They include senator Richard Burr, the Republican chairman of the Senate Intelligence Committee.

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The committee he oversees is one of several congressional panels investigating Russian interference during the election and possible collusion by Trump campaign staff.

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Senior Republican senator Lamar Alexander of Tennessee said: "It would have been easier to explain if the President had fired the FBI director earlier when Senator Schumer and other Democrats said they'd lost confidence in Mr Comey."

He added that the Senate must ensure, "the new FBI director is a person of unquestioned integrity".

The chief White House ethics lawyer during the George W Bush administration said Mr Comey's sacking would cause a political crisis.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 7 minutes 11 seconds 7 m Richard Painter on the sacking of FBI director James Comey

Richard Painter told the ABC's 7.30 program it was crucial Mr Trump appointed an independent prosecutor to continue the Russia investigation.

"If we don't have an independent prosecutor, no-one is going to trust the result of an investigation undertaken by an FBI supervised by President Trump and his Justice Department," he said.

"We are in a very, very serious situation right now."

Senator Marco Rubio, who lost to Mr Trump in the race to be the Republican presidential candidate last year, said: "I do have questions about why he was dismissed at this time given his service and performance."

Jeff Flake, a Republican senator from Arizona, who The Washington Post notes is up for re-election next year:

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And Mark Salter, a former senior adviser for 2008 Republican presidential candidate John McCain:

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ABC/wires