As many as 30 email accounts, some belonging to Hillary Clinton's top aides, handled "top secret" information discovered on the former secretary of state's private unsecured email server, reported Fox News.

An unnamed U.S. government source told the news outlet that accounts belonging to top Clinton aides Huma Abedin, Cheryl Mills, Jake Sullivan and Philippe Reines handled some of the 22 top secret messages. Highly sensitive information was also found on the account of State Department Under Secretary for Management Patrick Kennedy.

Another source told Fox that as many as 30 accounts may have transmitted the information.

The State Department has retroactively classified at lower levels more than 1,500 emails found on Clinton's server. For the first time last month, it confirmed that 22 were classified as top secret. The top secret messages were not released publicly, even in redacted form, because the information posed such a great risk to national security. At least one of the emails contained "oblique" references to CIA assets, according to the New York Times.

It was these top secret messages that were found on the accounts of at least a dozen of Clinton's aides, the official told Fox News.

The State Department said its Diplomatic Security and Intelligence and Research bureaus plan to investigate whether any of the information was classified at the time it was transmitted, according to the Associated Press.

The FBI confirmed to the State Department this week that it is investigating Clinton to determine if she mishandled classified information.

Clinton insists that she never sent information that was marked classified at the time, and has suggested Republicans are taking advantage of her email arrangement to hurt her presidential aspirations.