Former FBI deputy director Andrew McCabe has been sacked days before he could retire with pension rights.
He was fired by US Attorney General Jeff Sessions, who said an internal review found he leaked information and misled investigators.
Mr McCabe denied the claims and said he was being targeted because of his involvement in the inquiry into alleged Russian meddling in the 2016 election.
Mr Trump has long accused Mr McCabe of bias in favour of the Democrats.
He immediately praised Mr Sessions' decision to fire him.
In December, the president appeared to taunt the then number two at the FBI, when he tweeted: "FBI Deputy Director Andrew McCabe is racing the clock to retire with full benefits. 90 days to go?!!!"
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