Story highlights Former Defense Secretary Robert Gates questioned Hillary Clinton's judgment over a private email server

"I don't know what originally prompted her to think that was a good idea," he said

(CNN) Former Defense Secretary Bob Gates told Yahoo News' Katie Couric on Thursday that he questions Hillary Clinton's judgment in using a private email server while serving as secretary of state.

"There are obviously a lot of questions about her judgment and her policy positions subsequent to when I left" the cabinet, he said. "There's the whole email thing, which I think is really a concern in terms of her judgment."

Pressed on the matter, Gates added, "I don't know what originally prompted her to think that was a good idea."

"I never used email for official business, either as CIA director or secretary of defense," he continued. "... And so using an offline server, I think, was an error in judgment."

CNN has reached out to the Clinton campaign for response.

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