The inspector general’s report concluded that Mr Comey’s actions were not motivated by political bias but that the former bureau chief was insubordinate and departed from normal protocol numerous times.
“We found no evidence that the conclusions by the prosecutors were affected by bias or other improper considerations; rather, we determined that they were based on the prosecutors’ assessment of the facts, the law and past department practice,” the report said.
Mr Comey announced in the summer of 2016 that Mrs Clinton would not be charged with any crime, including the handling of classified material on a personal email address.
It was one of the most controversial investigations in modern FBI history and entangled the bureau, which usually stands apart from politics, in the 2016 presidential election.
The watchdog criticised Mr Comey for his unusual news conference in July 2016 at which he disclosed his recommendation against bringing charges. It was also unhappy with his decision, against the recommendation of the Justice Department, to reveal to Congress that the FBI was reopening the investigation following the discovery of new emails.
The inspector general also faulted the FBI for failing to act with more urgency in reviewing the new emails found on a former congressman’s laptop.
The report also says Mr Comey occasionally used personal email for work, including forwarding drafts of messages and other unclassified items.
Mr Comey said he wasn’t sure if that was in accordance with FBI regulations, but had checked it with another official and he “had the sense that it was okay.”
The inspector general says he did not follow regulations.
Shortly after the news that Mr Comey used his person email emerged, Hillary Clinton retweeted a post saying so, writing “But my emails.”
In reaction to the report, Mr Comey tweeted that he respects the inspector general’s work and believes the conclusions are “reasonable” but added that “people of good faith” can see the “unprecedented situation differently”.
Writing in the New York Times, Mr Comey said: “I do not agree with all of the inspector general’s conclusions, but I respect the work of his office and salute its professionalism.
“All of our leaders need to understand that accountability and transparency are essential to the functioning of our democracy, even when it involves criticism.”
President Donald Trump claims a politically biased bureau tried to undermine his campaign and, through the later Russia investigation, his presidency.
The White House said the report confirmed Mr Trump’s “suspicions” about Mr Comey and about the “political bias among some of the members of the FBI”.