Mr Comey, who had been leading an inquiry into alleged Russian meddling in the US election, was fired by the president on Tuesday.
Mr Trump has since insisted he was told by Mr Comey that he was not under investigation.
Mr Trump said he had been told twice by Mr Comey over dinner and once over the phone that he was not a target of the inquiry.
The president said: "I said: 'If it's possible, would you let me know: am I under investigation?' He said: 'You are not under investigation.'"
The FBI investigation is looking into possible collusion between Trump election campaign officials and Moscow.
Mr Trump said this week he alone was responsible for the decision to sack Mr Comey, calling him a "showboat" and "grandstander".
Mr Trump has dismissed the FBI investigation as a "charade" and has said Democrats are using "fake news" about collusion with the Russians as an excuse for losing the election.
In another tweet on Friday, Mr Trump said: "When [former Director of National Intelligence] James Clapper himself, and virtually everyone else with knowledge of the witch hunt, says there is no collusion, when does it end?"
However, Mr Comey's successor, acting FBI Director Andrew McCabe, said on Thursday that it remained "a highly significant investigation".
In testimony to the Senate Intelligence Committee, he also cast doubt on White House claims that Mr Comey had lost the confidence of his staff.
"I can confidently tell you that the vast majority of employees enjoyed a deep and positive connection to Director Comey," Mr McCabe said.
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