(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.
Gates also said his views began to diverge with Clinton's over U.S. involvement in Libya in 2011, but that his opinion of her overall is a "mixed picture."
Earlier this month, Gates told ABC's Martha Raddatz there is an over-classification problem in government, seemingly backing Clinton's position on the matter.
Gates became defense secretary under then-President George W. Bush in 2006 but continued his position during Obama's first term until 2011, where he overlapped with Clinton, who served as secretary of state from 2009 to 2013.