Former Defense Secretary Robert Gates on Thursday said he believed former Secretary of State Hillary Clinton's secret email server was hacked by either China, Iran or Russia.

'I think the odds are pretty high,' Gates said in an interview with Hugh Hewitt. 'The Pentagon acknowledges that they get attacked about 100,000 times a day.'

Gates, who led the CIA from 1991 to 1993 and the Pentagon from 2006 to 2011, was asked if he was surprised by continuing news about Clinton's private, home-based email server and the allegation by an inspector general that it contained highly classified information.

Gates said he was.

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Former Defense Secretary Robert Gates said in a radio interview on Thursday that former Secretary of State Hillary Clinton's email server was most likely hacked by Russia, China or Iran

'Yeah, that’s a concern for me,' he said. 'I never used email when I was head of CIA or head of the Department of Defense.'

'As I used to joke, I didn’t want to have some chief of station overseas email me and say he was going to do something if I didn’t get back to him in three hours, and I would get back from a five-hour hearing to discover I was two hours too late.'

'I preferred dealing with people face to face and putting a signature on a piece of paper on matters of real national security and importance.'

Hewitt later asked Gates if national security could have been compromised if foreign countries did hack Clinton's server.

'It would depend entirely on what she put on there. And I just, I haven’t read any of these emails, so I don’t know what was on those servers,' Gates said.

Gates is promoting a new book, 'A Passion For Leadership.'

The former secretary of state and current Democratic presidential front-runner was campaigning in Burlington, Iowa, on Wednesday