Hillary Clinton broke her silence on the FBI's investigation into her email system Friday, arguing FBI Director James Comey had simply been "speculating" when he testified before Congress that her private server was likely hacked by hostile agents.

"I just believed that the material that was being communicated by professionals ... they did not believe that it was [classified]," Clinton said during an interview with CNN's Wolf Blitzer. "I did not have a basis for second-guessing their conclusion and these [emails] were not marked. They were not marked."

Clinton's claim that nothing on her server was marked classified, which she repeated in a subsequent interview with NBC's Lester Holt Friday, was a direct contradiction to Comey's statement that his agents had indeed discovered three emails that bore classification markings.

"He clarified as to the State Department that the very few no more than three documents that they thought might have some kind of marking," Clinton said. "Two of them were the result of human error."

In defense of the FBI's discovery of the marked emails, State Department officials argued earlier this week that the markings had appeared on the documents by accident.

Clinton told Holt the FBI director had been "speculating" when he said her server was vulnerable to hackers.

"There is no evidence that the system was breached or hacked successfully," she said.