FBI Director James Comey said he doubted whether Hillary Clinton was “sophisticated enough” to recognize the classified markings on emails sent to her private server during a congressional hearing Thursday.

“I think it’s possible that she didn’t understand what a ‘C’ meant when she saw it in the body of an email,” Comey said of the “confidential” markings in some of the classified emails that ended up on an unsecured server in Clinton’s basement.