Former FBI Director James Comey made a "serious error of judgment" when he announced shortly before the 2016 US presidential election that he was reopening an investigation into candidate Hillary Clinton's use of a private e-mail server, the Justice Department's internal watchdog said on Thursday.

A 500-page report by Inspector General Michael Horowitz concluded that Comey was "insubordinate" in handling the probe into Clinton, damaging the bureau and the Justice Department's image of impartiality even though he didn't exhibit any political bias or try to influence the election.

Horowitz, whose office said it reviewed more than 1.2 million documents and interviewed more than 100 witnesses, didn't challenge Comey's fundamental decision against recommending prosecution of Clinton for mishandling classified information.

But it criticized him for violating Justice Department policies and accused him of usurping the authority of Attorney General Loretta Lynch when in July 2016 he announced there would be no charges against Clinton for her email use as secretary of state.

In a response to the report, Comey said in a tweet that he disagrees with some of its conclusions, but that he respects the inspector general's work and believes the conclusions are "reasonable." Comey said "people of good faith" can see the "unprecedented situation differently."

Horowitz said that five FBI officials expressed hostility toward Trump before his election as president and disclosed in his report to Congress that their actions have been referred to the bureau for possible disciplinary action.

"The president was briefed on the IG report earlier today, and it reaffirmed the president's suspicions about Comey's conduct and the political bias among some of the members of the FBI," White House press secretary Sarah Huckabee Sanders said of the report.

The FBI said in a statement included in the inspector general's report that Comey's handling of the Clinton findings may have violated regulations on releasing information and that his letter disclosing reopening of the inquiry shortly before the election "was a serious error in judgment."

Reuters/Bloomberg