Democratic presidential front-runner Hillary Clinton has tried to portray the recently-announced FBI probe into the former secretary of state's exclusive use of an unsecured email system as a simple fact-finding mission, but sources close to the investigation told the New York Post on Wednesday that the investigation is indeed a full-fledged criminal probe.

The FBI is investigating the extent to which Clinton used her private email and home server to send and store classified documents, the source said.

"It's definitely a criminal probe. I'm not sure why they're not calling it a criminal probe," the source told the Post.

"The DOJ [Department of Justice] and FBI can conduct civil investigations in very limited circumstances," the source said, stressing that it's not a mere civil investigation. "In this case, a security violation would lead to criminal charges. Maybe DOJ is trying to protect her campaign."

It was revealed earlier this year that Clinton, against State Department guidelines, exclusively used a private email account and home-based server to conduct all official federal business during her tenure as secretary of state. Numerous private sector experts have said that they are convinced that Clinton's unsecured system was compromised by hackers.

The New York Times reported last month that two inspectors general asked the Justice Department to open a "criminal" investigation into Clinton's email use. The Clinton camp quickly persuaded the Times to change its original story, and it removed the "criminal" description in favor of more neutral language.

"It is now more clear than ever that the New York Times report claiming there is a criminal inquiry sought in Hillary Clinton's use of email is false," Clinton spokesperson Nick Merrill said in a statement in July, reported MSNBC. "It has now been discredited both by the Justice Department and the Ranking Member of the House Oversight Committee. This incident shows the danger of relying on reckless, inaccurate leaks from partisan sources."

Clinton has repeatedly insisted that she did not send or receive any information that was classified at the time. However, the inspectors general recently examined a random limited selection of 40 of her emails and discovered that four of them contained classified information, which "were classified when they were sent and are classified now," spokeswoman Andrea Williams told CNN. State Department officials told the inspectors general that "there are potentially hundreds of classified emails within the approximately 30,000 provided by former Secretary Clinton."

If investigators prove "that Clinton 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," former federal prosecutor Bradley Simon told the Post.

"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," he said.

Clinton's email probe lawyer, David Kendall, also represented Petraeus, who was charged earlier this year for providing classified information to his mistress biographer.

Makan Delrahim, former chief counsel to the Senate Judiciary Committee, who also served as a deputy assistant secretary in President Bush's DOJ, told the Post that he believes investigators are "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."

Clinton's team has not yet addressed the sources' claims.