US Attorney General Jeff Sessions has fired FBI official Andrew McCabe, who had been accused of political bias by President Donald Trump.
In January Mr McCabe resigned as deputy director and was placed on leave.
He had been deeply involved in the FBI investigations into Hillary Clinton's use of email and Russia's alleged meddling in the presidential campaign.
He was sacked just two days before he was expected to retire, and could lose some of his pension rights.
In a statement issued late on Friday Mr Sessions said: "Based on the report of the Inspector General, the findings of the FBI Office of Professional Responsibility, and the recommendation of the Department's senior career official, I have terminated the employment of Andrew McCabe effective immediately."
Mr McCabe responded by saying he was being "singled out" because of the role he played in the aftermath of the firing of last year of then-FBI director James Comey.