FBI Director James Comey on Thursday said it was not clear whether Hillary ClintonHillary Diane Rodham ClintonThe Memo: Polls flash warning signs for Trump Polls suggest Sanders may be underestimated 10 declassified Russia collusion revelations that could rock Washington this fall MORE fully understood the government’s marking system for classified messages.

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During a House Oversight Committee hearing, Rep. Mark Meadows (R-N.C.) pressed Comey about whether he was testifying that Clinton didn’t know what a classified marking is.

“No, not that she would have no idea what a classified marking would be,” Comey responded. “It’s an interesting question whether she … was actually sophisticated enough to understand what a C in [parentheses] means.”

“You asked me if I would assume someone would know,” he added. “Probably before this investigation, I would have. I am not so sure of that any longer. I think it’s possible — possible — that she didn’t know what a C meant when she saw it in the body of an email like that.”

Emails containing sensitive information are supposed to be marked at the top of a message or in the subject line to indicate that the contents are classified. 

The press secretary for Clinton's presidential campaign seized on Comey’s testimony as evidence the former secretary of State did not intentionally lie when she said there were no emails marked classified on her private server.

“Director Comey acknowledges the so-called ‘marked’ emails were not properly marked in a way one could recognize them as classified,” Brian Fallon, a former Justice Department spokesman, tweeted.