Former FBI boss Robert Mueller has been named as a special prosecutor to oversee a probe into Russia’s alleged interference in the US election.
Deputy Attorney General Rod Rosenstein said it was “in the public interest” to bring an outsider in to oversee the investigation.
Calls for a special prosecutor have mounted since President Donald Trump fired the FBI director last week.
Mr Mueller is a former prosecutor who served as FBI chief from 2001 to 2013.
“Based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command,” Mr Rosenstein said in a statement.
The FBI investigation is also looking into potential links between Mr Trump’s campaign team and Russia.
But Mr Rosenstein added that his decision was not a “finding that crimes have been committed or that any prosecution is warranted. I have made no such determination”.
It is unclear if Mr Trump played a part in the decision.
The White House has been engulfed in controversy following a string of controversies including Mr Comey’s abrupt dismissal and allegations that Mr Trump asked the ousted FBI chief to drop an inquiry into links between his ex-national security adviser and Russia.
Mr Trump’s national security adviser Michael Flynn was forced out in February after he misled the vice-president about his conversations with Russia’s ambassador before Mr Trump took office.
The White House has denied it put pressure on Mr Comey but the revelations fuelled Democratic claims that Mr Trump tried to conceal his team’s connection to Russia.
Mr Mueller, 72, served as FBI director under Presidents George W Bush and Barack Obama as the longest serving bureau chief since J Edgar Hoover.
He is expected to announce his resignation from a private law firm to avoid conflicts of interest.