An attorney general from the George W. Bush administration said Sunday the idea that Hillary Clinton did not transmit a high volume of classified material on her email server is "inconceivable."

"So far as definitions of what's classified and what's not, obviously, at the margins, there are some things that may or may not be classified, but we're talking about information that went to the secretary of state, who is the highest foreign relations officer of the United States," said Michael Mukasey, former attorney general, on "Fox News Sunday."

"It's inconceivable to me that a great deal of that was not classified," he added.

Mukasey dismissed the Clinton campaign's claims that Clinton is not the subject of a criminal investigation.

Her aides fiercely contested a recent New York Times story that suggested the Justice Department had opened a criminal probe into Clinton's email activity.

"The Justice Department itself has said this is a non-criminal inquiry," said spokesman Brian Fallon in a campaign video released Friday. "Hillary Clinton herself is not the target."

But Mukasey noted the FBI could not be looking into Clinton's emails without looking into the former secretary of state herself.

"To say that the investigation is not of someone personally is ridiculous," Mukasey said. "The FBI does not investigate machines. It investigates people. And she's certainly one of the people who's being investigated."

The former attorney general expressed "confidence" in the Justice Department's investigation of the server.