However, Inspector General Michael Horowitz also concluded in a long-awaited, 500-page report that Comey did not exhibit any political bias in his actions or try to influence the election, and does not contest the decision not to prosecute Clinton for the e-mail affair.
A long-serving law enforcement official, Comey became a controversial figure in the 2016 election, drawing accusations from both Republicans and Democrats that his handling of an investigation into Clinton’s e-mails influenced the campaign.
Comey later headed a separate probe into alleged ties between President Donald Trump’s campaign and Russia.
Trump fired him as head of the Federal Bureau of Investigation (FBI) in 2017 and has frequently criticised him since.
Both sides of the partisan divide in US politics are expected to use the Horowitz report to press their cases against Comey.
The inspector general’s inquiry has focused on public statements made by Comey about Democrat Clinton’s use of a private e-mail server, instead of a State Department server, while she was secretary of state.
In October 2016, less than two weeks before Election Day, Comey sent members of Congress a letter disclosing that a probe into Clinton’s e-mails was being reopened after new e-mails were found.
Clinton has said that Comey’s letter contributed to her unexpected defeat by Trump.
Two days before the November 8 election, Comey said that the FBI had found no additional evidence in the new e-mails.
In an op-ed published in the New York Times, Comey said that the report confirmed the call there was “no prosecutable case” against Clinton, and stood by his decision to inform Congress in that the case had been reopened days before the election.
“I chose to speak and tell the truth,” Comey wrote.
John Podesta, who ran the Clinton campaign, said “the report demonstrates beyond doubt” that Comey was unfair to Clinton by announcing developments of the e-mail investigation during the campaign while not revealing the presence of the separate probe beginning in July 2016 into the Trump campaign and Russia.
“This report confirms what we have known for a long time – that the FBI inappropriately applied a double standard to the Clinton and Trump investigations, which hurt her and helped elect him,” Podesta told Reuters.
The report sharply criticised Comey for violating Justice Department policies and accused him of usurping the authority of then attorney general Loretta Lynch when in July 2016 he announced there would be no charges against Clinton for her e-mail use as secretary of state.
Trump and his allies have accused a clique of FBI and Justice Department officials of working against Trump.
The Horowitz report was highly critical of Peter Strzok and Lisa Page, two FBI staff members who exchanged highly charged political messages, finding their texts cast a cloud over the FBI and created the appearance of bias.
In July 2016, Comey held an unusual news conference to explain why the FBI would not be recommending criminal charges against Clinton over her use of the private server for some official business.
He chastised Clinton for being “extremely careless” but said there was insufficient evidence to charge her with a federal crime.