FBI Director James Comey said Thursday that Hillary Clinton should have known not to transmit classified information on her private server.

"I think she was negligent," Comey said during a House Oversight and Government Reform Committee hearing.

While the federal statute dictating how classified information should be handled specifically mentions "gross negligence" as an illegal activity, Comey said he did not find enough evidence to prove "beyond a reasonable doubt" that Clinton violated the law.

Much of the hearing Thursday focused on the legal question of whether intent is necessary to bring criminal charges against an individual who mishandles classified information.

Comey said he did not recommend an indictment in the case because he did not find enough evidence to suggest Clinton knowingly exposed classified intelligence to hostile agents by housing it on an unsecured server network.

"There's evidence of that," Comey admitted when pressed on whether Clinton should have known which emails were classified and where those records should have been stored.

But Comey repeatedly argued he did not find enough details to prove definitively that Clinton and her aides acted maliciously.

The FBI director said he was not sure whether Clinton was "sophisticated enough" to recognize classified markings.