A former U.S. attorney said the FBI's reported investigation of "public corruption" at the Clinton Foundation has actually been going on for months, although it made headlines for the first time Monday.

Joseph DiGenova, who was appointed by President Ronald Reagan, said the FBI's foundation probe "has reached such a proportion of inquiry that it's starting to sneak out," but that it was sparked months ago by information pulled from Hillary Clinton's private server and from the tax filings of corporations that have donated to the charity.

"There are now, I am told, 150 agents working on this case," DiGenova told the Washington Examiner Monday, noting that was "a very unusually high number" of investigators to be working on one case.

Talk of Clinton's legal woes returned in full force Monday with reports that the FBI has expanded its investigation to include the alleged exchange of favors between Clinton Foundation insiders and State Department leadership under Clinton.

The reports came just days after the State Department released a controversial email that suggested Clinton had asked an aide to remove the classification markings from a document and "send it nonsecure."

A spokesman for the Clinton Foundation did not return a request for comment about the new reports.

The Justice Department did not respond to a request for comment on reports that the FBI had expanded its investigation.

But DiGenova said the FBI "is making very substantial progress" in its investigation of the "public corruption" allegations against the Clinton Foundation. He said the bureau is presently preparing subpoenas for the foundation's financial records, among other pieces of evidence.

"This was inevitable given the discussions that have appeared over the past few months concerning contributions which were done in tandem with requests for officials acts," DiGenova said.

"This makes the Bob McDonnell case look like a nothing burger," he added. DiGenova was referring to former Republican Gov. Robert McDonnell of Virginia, who was sentenced in Jan. 2015 to two years in prison for public corruption.

Andrew Herman, a government ethics attorney at Miller & Chevalier, also likened the public corruption allegations against Clinton and her family's foundation to the McDonnell case.

"This is like many previous cases if there's something there," he said. "If there was use of the position for private gain for the foundation, then you're looking at a Menendez-type case, or [a case like that of] Bob McDonnell."

Herman was referring to the April 2015 indictment of Sen. Robert Menendez, D-N.J., on corruption charges stemming from allegations that he helped a friend in exchange for kickbacks.

He said agents would need to uncover evidence that Clinton herself helped donors to the charity in order for the Justice Department to bring public corruption charges against her.

"They'd have to demonstrate that there was a direct tie between some kind of official act that Secretary Clinton engaged in and contributions to the foundation, or some other way that she gained," Herman said.

Although he said the allegations against the foundation are "significant," he noted "tensions" within the Justice Department could ultimately smother the probe.

"As is often the case, the investigators, the agents may have a certain view of things, but they're not the ones who are going to make decisions about how to proceed," Herman said. "I wonder how they can resolve that internally within the Justice Department, if it's even possible to resolve it within the Justice Department, given the tensions and the various conflicting factors and the fact that this is an administration in which, until very recently, Secretary Clinton was a cabinet member."

DiGenova also said the Justice Department may be reluctant to bring charges against Clinton given her political status, but argued Attorney General Loretta Lynch will have "no choice" but to indict the former secretary of state in the face of enormous pressure from within the FBI.

While the recommendations for prosecution would come in the form of a confidential memo from the FBI director to the attorney general, "the bureau will no doubt let it be known" that such a memo has been sent in order to put additional pressure on Lynch to indict Clinton, DiGenova said.

He said the latest spate of leaks about the public corruption investigation is what the FBI calls "shaking the tree," or making information about a probe public in order to encourage witnesses.

The FBI investigation into Clinton's private email network began last year after a pair of inspectors general found evidence that at least two emails on her server should have been classified the moment they were sent.

Since then, the probe has reportedly mushroomed to include allegations of obstruction of justice, potential violations of the Espionage Act and, most recently, possible public corruption involving the Clinton Foundation.

The FBI has remained tight-lipped about the official status of its investigation, pointing to its long-standing policy of declining to comment on ongoing investigations.

But the reported expansion of an already serious probe could spell trouble for Clinton just weeks ahead of the Iowa caucuses.