The U.S. Justice Department's investigation of Fox News has widened to include a second law enforcement agency.

Financial crimes experts from the United States Postal Inspection Service are now involved, according to four sources connected to the investigation.

Mail fraud and wire fraud cases are part of the USPIS purview.

Investigators from both the USPIS and the Justice Department have been conducting interviews in recent weeks -- including with some former Fox staffers -- to obtain more information about the network's managers and business practices, the sources said.

The existence of the federal investigation was revealed in February. At the time Fox News and its parent company 21st Century Fox said they had not been subpoenaed, but a spokeswoman said, "we have been in communication with the U.S. Attorney's office for months — we have and will continue to cooperate on all inquiries with any interested authorities."

Contacted about this story, the Justice Department and the USPIS had no comment. 21st Century Fox also declined to comment.

In February the investigation was reported to be focusing on settlements made with women who alleged sexual harassment by former Fox News boss Roger Ailes, and questions about whether Fox had a duty to inform shareholders about the settlement payments.

The investigators have been asking "how the shareholder money was spent; who knew; and who should have known," one of the sources said.

But, CNNMoney has learned, the settlement payments are not the only thing they are examining.

Investigators have been probing possible misconduct by Fox News personnel and asking questions about the overall environment at the network.

Investigators have also been asking questions about mysterious confidants of Ailes -- people who were known inside Fox as "friends of Roger."

Ailes, through Fox News, employed a number of longtime friends as consultants, for purposes that remain unknown. One of the consultants earned $10,000 a month by submitting a monthly invoice to Fox.

When Ailes was forced out of Fox News last July, some of the consultants were also dismissed.

"Longtime friends of Roger's dating back to the '70s are beginning to exit the building," a source told CNNMoney in August.

Some of the people interviewed by investigators have been asked about these "friends of Roger" and what they did behind the scenes at Fox.

While the Justice Department has not commented on the investigation, it is clear, according to the sources connected to it, that Ailes and settlement payments are only one part of the probe.

The sources asked for anonymity because of the sensitivity of the matter.

The probe began under the Obama administration. Attorney Judd Burstein, who represents some of Ailes' accusers, said in February that the U.S. Attorney's Office for the Southern District of New York was conducting the investigation.

That office was led by U.S. attorney Preet Bharara until March, when President Trump fired him after he refused to resign. Dozens of other U.S. attorneys who were Obama appointees were asked to resign at the same time.

Bharara's departure has stirred speculation about whether the Trump administration's Justice Department will follow through on any findings involving Fox, Trump's favorite news network.

When asked by CNNMoney this week if he could discuss the Fox probe, Bharara said, "I'm not talking about any investigations that may or may not" have taken place.