The FBI has been left reeling by US President Donald Trump's unexpected firing of its director James Comey, with agency staff scheduling an emergency high-level meeting amid speculation about who would replace Comey in the top job.
An FBI official, who was not authorised to speak to reporters and so asked not to be identified, said the staff meeting would meet late on Tuesday night (local time) to explore the next steps for the law-enforcement agency.
Trump sacked Comey amidst a probe into the Trump 2016 presidential campaign's possible collusion with Russia to influence the election outcome, sparking backlash from some Democrats in Congress who said the decision had the appearance of a cover-up and some Republicans who called the timing troubling.
The official said the FBI had no idea the Trump administration was considering dismissing Comey and the news "took even top officials by surprise."
An FBI spokesman did not respond to a request for comment.
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Deputy Director Andrew McCabe was expected to step in for now, but he was unlikely to be nominated by Trump for the director's post, said two former FBI officials.
Comey, who was appointed by former Democratic president Barack Obama, had six and a half years left in his term as director of an agency with 56 US field offices and more than 30,000 employees.
The new director must be appointed by Trump and confirmed by the US Senate.
Comey is the third high-profile law-enforcement official to be fired under Trump.
Preet Bharara was fired as US attorney for the Southern District of New York and Sally Yates was fired as acting attorney general.
Trump, a Republican businessman and former TV celebrity, was elected on November 8 and took office on January 20.
One possible contender for director of the FBI is Dana Boente, No. 3 at the Justice Department and former acting attorney general, said the two former FBI officials.
Other potential choices include Republican Representative Trey Gowdy, a former prosecutor who led a congressional inquiry into former secretary of state and 2016 Democratic presidential nominee Hillary Clinton's role in the 2012 attacks on the US Consulate in Benghazi, Libya.
