The feds are investigating to what extent Clinton relied on her home server and other private devices to send and store classified documents, according to a federal source with knowledge of the inquiry.

“It’s definitely a criminal probe,” said the source. “I’m not sure why they’re not calling it a criminal probe.

“The DOJ [Department of Justice] and FBI can conduct civil investigations in very limited circumstances,” but that’s not what this is, the source stressed. “In this case, a security violation would lead to criminal charges. Maybe DOJ is trying to protect her campaign.”

Clinton’s camp has downplayed the inquiry as civil and fact-finding in nature. Clinton herself has said she is “confident” that she never knowingly sent or received anything that was classified.

The inspector general for the intelligence community has told Congress that of 40 Clinton e-mails randomly reviewed as a sample of her correspondence as secretary of state, four contained classified information.

If Clinton is proven to have knowingly sent, received or stored classified information in an unauthorized location, she risks prosecution under the same misdemeanor federal security statute used to prosecute former CIA Director Gen. David Petraeus, said former federal prosecutor Bradley Simon.

The statute — which was also used to prosecute Bill Clinton’s national security adviser, Sandy Berger, in 2005, is rarely used and would be subject to the discretion of the attorney general.

Still, “They didn’t hesitate to charge Gen. Petraeus with doing the same thing, downloading documents that are classified,” Simon said. “The threshold under the statute is not high — they only need to prove there was an unauthorized removal and retention” of classified material, he said.

Clinton’s lawyer in the e-mail probe is longtime Bill Clinton attorney David Kendall, who also repped Petraeus, who pled guilty earlier this year to providing classified documents to his mistress biographer.

“My guess is they’re looking to see if there’s been either any breach of that data that’s gone into the wrong hands [in Clinton’s case], through their counter-intelligence group, or they are looking to see if a crime has been committed,” said Makin Delrahim, former chief counsel to the Senate Judiciary Committee, who served as a deputy assistant secretary in the Bush DOJ.

“They’re not in the business of providing advisory security services,” Delrahim said of the FBI. “This is real.”

The Clinton campaign did not immediately respond to a request for comment.