Donald Trump has fired FBI Director James Comey as he leads the probe into whether the Trump campaign had links to Russian efforts to undermine last year's presidential election.
Mr Trump said he took the decision because of Mr Comey's handling of the Hillary Clinton email scandal, which she claimed cost her victory in the election.
"Today [Mr Trump] informed FBI Director James Comey that he has been terminated and removed from office," a statement from Press Secretary Sean Spicer said.
"President Trump acted based on the clear recommendations of both Deputy Attorney-General Rod Rosenstein and Attorney-General Jeff Sessions."
In a letter to Mr Comey the president said the firing was necessary to restore "public trust and confidence" in the FBI.
"While I greatly appreciate you informing me, on three separate occasions, that I am not under investigation, I nevertheless concur with the judgement of the Department of Justice that you are not able to effectively lead the bureau," Mr Trump wrote.
Mr Comey was criticised for revealing that then Democrat presidential candidate Hillary Clinton was being investigated by the FBI for her use of a private email server while secretary of state.
Some commentators have questioned the timing of Mr Trump's decision as the facts of that probe were known when Mr Trump assumed and asked Comey to remain at the helm of the FBI.
Mr Trump had even praised the FBI director's handling of the email probe on numerous occasions, saying that it "took guts" for Mr Comey to reveal that Ms Clinton was under investigation during the dying days of the presidential campaign.
Mr Trump will now be able to nominate Mr Comey's successor, who could resume leadership of the investigation into any Trump campaign links to Russia.
Democrats have called Mr Comey's firing an effort to neuter the Russia probe and have called for a special prosecutor to take charge of it.