The Justice Department probe into Hillary Clinton's private email server has so far been limited to investigating whether classified information was leaked during her tenure as secretary of state, but the inquiry will likely widen as authorities take a look at how other former government officials may have been affected.

According to NPR, agents will begin questioning close associates of Clinton at the State Department and beyond.

"I think that the FBI will be moving with all deliberate speed to determine whether there were serious breaches of national security here," Ron Hosko, the former head of the FBI's criminal investigative division, told NPR.

"I would want to know how did this occur to begin with, who knew, who approved."

Authorities are trying to discern whether Clinton and her aides mishandled secrets about the Benghazi terrorist attacks and other top secret information by corresponding about them by email.

Clinton has insisted that she did not use her private email account to send or receive any material considered classified.

"Whether it was a personal account or a government account, I did not send classified material, and I did not receive any material that was marked or designated classified which is the way you know whether something is," she said at a Tuesday press conference.

Two lawyers told NPR that a formal criminal investigation is being considered and could be launched soon, though Clinton may not be the target.

The Clinton campaign has insisted that she is clear of any wrongdoing and that the government inquiry would be limited to a "security-related review."

"She was at worst a passive recipient of unwitting information that subsequently became deemed as classified," Brian Fallon, Clinton campaign press secretary, told reporters, according to NPR.

"When it comes to classified information, the standards are not at all black and white, and in the absence of markings that officially designate something classified, reasonable people each taking their responsibilities extremely seriously, can nonetheless disagree on the character of the information they are dealing with — and both could be completely justified in that perspective."

He added, "And that is why we are so confident that this review will remain a security-related review. We think that furthermore this matter is mostly just shining a spotlight on a culture of classification that exists within certain corners of the government, especially the intelligence community."

In an interview for Newsmax TV, Michael Mukasey, former attorney general for George W. Bush, said the government has to meet a high burden of proof.

"They'd have to show that she was responsible for having the information on that server and essentially knew what was on there," Mukasey said.