Beyond the Democratic front-runner, the probe is most likely examining confidants Cheryl Mills, Jake Sullivan and Huma Abedin.

The most serious is loss of security clearances, which could complicate Clinton's naming of a national security team if she becomes president.

Clinton was secretary of state until early 2013. Most of her top advisers left shortly thereafter.

FBI Director James Comey said at a congressional hearing on Thursday that his investigators found "evidence of mishandling" of classified information, though they found that neither Clinton nor her aides intended to do wrong and thus could not be charged with a crime. Of Clinton in particular, he said: "I think she was extremely careless. I think she was negligent."

Comey declined to say what penalty Clinton might face if a criminal charge wasn't appropriate - asserting that his investigation was focused on whether the misconduct that occurred constituted a violation of the law. But said if a bureau employee mishandled classified information, that person would face a review and could be reprimanded, suspended or even fired.

"They would face consequences for this," he said.

It is not clear how long the State Department review will take, and whether the results will be made public.

Comey said on Tuesday that his investigators had no "direct evidence" that Hillary Clinton's email account had been "successfully hacked," both private experts and federal investigators immediately understood his meaning: It very likely had been breached, but the intruders were far too skilled to leave evidence of their work.