The full impact of FBI director James Comey's stunning announcement about restarting a probe into former secretary of state Hillary Clinton's use of a private e-mail server is not yet clear, but he is already coming under heavy fire.

His decision to make the vague disclosure last Friday has made him the target of dozens of officials and analysts from both sides of the aisle.

At the heart of the issue is that he may have turned a supposedly neutral Federal Bureau of Investigation (FBI) into a political player.

As Fox News contributor, Judge Jeanine Pirro, put it: "Now, you know I support Donald Trump and I want him to win. But whether it's Hillary Clinton or anyone else, Comey's actions violate not only longstanding Justice Department policy... but even more important, the most fundamental rules of fairness and impartiality."

Former government officials point out that the Department of Justice - which the FBI is part of - has traditionally advised its staff to avoid any appearance of meddling in elections. The practice was even formalised in a 2012 memo by then Attorney-General Eric Holder. As reported by The New Yorker, Mr Holder wrote that officials "must be particularly sensitive to safeguarding the department's reputation for fairness, neutrality, and non-partisanship". Even outside of campaigns, it has been the FBI's practice not to comment on investigations if no charges are filed. Mr Comey violated that practice when he called Mrs Clinton "extremely careless" in July, and then compounded that problem with his latest statement.