The Federal Bureau of Investigation probe into the security of the private email system Hillary Clinton used while secretary of state has been labeled "criminal."

The New York Post reported that a federal source close to the investigation defined it as "definitely a criminal probe," expressing uncertainty at why the FBI has refrained from calling it such.

"The DOJ [Department of Justice] and FBI can conduct civil investigations in very limited circumstances," explained the source. "In this case, a security violation would lead to criminal charges. Maybe DOJ is trying to protect her campaign."

Amid revelations that multiple emails contained on Clinton’s private system contained classified information, FBI officials over the past week contacted the Denver-based technology firm Platte River Networks, which helped manage the former secretary of state’s personal system.

The FBI also contacted Clinton attorney David Kendall regarding the security of the computer thumb drive in his possession that houses the 30,000 work-related emails that Clinton turned over to the State Department earlier this year in the wake of reports that she used a personal email system during her time there.

Two weeks ago, intelligence community Inspector General I. Charles McCullough concluded that five Clinton emails out of a sample of 40 he was permitted to review contained classified information. The messages in question allegedly held information from five different U.S. intelligence agencies.

McCullough referred the matter to the Department of Justice.

If found to have knowingly sent, stored or received classified information in a location unauthorized by the government, Clinton could face prosecution.

Clinton campaign spokesman Nick Merrill in a statement Wednesday maintained that the Democratic presidential candidate "did not send nor receive any emails that were marked classified at the time."

While he confirmed the probe, Kendall downplayed it, claiming in a statement, "Quite predictably, after the [Intelligence Community inspector general] made a referral to ensure that materials remain properly stored, the government is seeking assurance about the storage of those materials. We are actively cooperating."

The Democratic presidential candidate has herself repeatedly expressed confidence that she did not send or receive classified information over her private system, which she used exclusively during her time in the Obama administration.

Clinton is currently viewed as untrustworthy by a significant majority of American voters.