State Department building
State Department buildingThinkstock

The State Department is re-opening an internal investigation into whether Democratic presidential candidate Hillary Clinton and her top aides mishandled classified information, Fox News reported Thursday.

The investigation, which was first reported by The Associated Press, focuses on how classified emails to and from Clinton's private server were categorized at the time they were sent.

The State Department started its review in January after declaring 22 emails from Clinton's private server to be "top secret."

The investigation was halted after the FBI began investigating Clinton's private email server. On Wednesday, Attorney General Loretta Lynch said there would be no indictments resulting from the FBI probe, a day after FBI chief James Comey said he would recommend that Clinton not be indicted over the email saga.

"Given the Department of Justice has now made its announcement, the State Department intends to conduct its internal review," State Department spokesman John Kirby said in a statement quoted by Fox News.

"Our goal will be to be as transparent as possible about our results, while complying with our various legal obligations," he added.

Kirby set no deadline for the investigation's completion.

Clinton was Secretary of State until early 2013. Most of her top advisers left shortly thereafter.

Kirby said earlier this week that former officials can still face "administrative sanctions." The most serious of those penalties is loss of security clearances, which could complicate Clinton's naming of a national security team if she becomes president.

Beyond the Democratic frontrunner, the probe is most likely examining confidants Cheryl Mills, Jake Sullivan and Huma Abedin, who wrote many of the emails to their boss that the various investigations have focused on, noted Fox News.

The State Department said it would not identify former officials that still hold security clearances. But in an email Fox News made public in February, the department described Mills as still holding a valid clearance.

Clinton has said she used private emails out of "convenience", though she has also admitted it "would have been better" to have two accounts to separate work and personal emails.