America’s largest chain of psychiatric hospitals is the target of a multi-agency federal investigation into whether it systematically holds patients longer than necessary to maximize revenues — an allegation two nurses at one of its facilities raised following a protest at its headquarters in Pennsylvania last week.

According to three sources with direct knowledge of the investigation, officials are examining whether Universal Health Services directs its hospitals to hold patients for as many days as their insurer agrees to pay for, regardless of actual medical need. The probe has been ongoing since at least 2013, when the Department of Health and Human Services issued subpoenas to 10 UHS psychiatric hospitals.

But BuzzFeed News has exclusively learned that the investigation has since broadened to include the FBI and the Department of Defense, which is scrutinizing UHS’s billings to Tricare, the insurance plan for active military and their families. UHS, a $12 billion company, made nearly one-third of its revenues last year from government insurance providers such as Medicare and Medicaid.

Officials working on the current investigation continue to seek witnesses to any alleged fraudulent activity. “Putting together a successful prosecution will require the testimony of patients, intake coordinators, nurses, social workers, providers, and executives,” said one agent on the case.

The allegations against the company were raised during its shareholders meeting last week — both by the nurses who protested outside the event and by an investor in the room. The New York City Comptroller’s office, representing pension funds that own more than $25 million in UHS stock, cited ongoing investigations into the company when calling for it to abandon its shareholders voting system. An investment group for the union coalition Change to Win also voiced similar governance concerns in a 12-page letter to shareholders before the meeting.

The multiple-class voting system gives Alan Miller, CEO and chairperson, more than 80% of voting power, despite owning less than 15% of its total shares, the comptroller’s office complained.

“Insiders have total control of Universal Health Services, despite owning just a small fraction of the company,” NYC Comptroller Scott M. Stringer told BuzzFeed News in a statement. ”That puts every investor — including the New York City Pension Funds — at risk. From federal criminal investigations to damning exposés, it’s clear why shareowners are calling for change.”

The proposal was rejected by more than 90% of votes cast.

As the shareholders meeting proceeded, nurses from a nearby UHS-owned facility in Pennsylvania staged a protest outside, complaining of issues including understaffing and unsafe working conditions. A recent OSHA citation that found its workers are exposed to “serious physical injuries such as from bites, bruises or strains, sprains,” which underscored the employees’ concerns. (UHS is contesting the findings.)

Two nurses from the facility, Brooke Glen Behavioral Hospital, said they had direct experience with the company holding patients longer than necessary to collect higher insurance payments. They recalled telling doctors that patients were safe to be discharged, but that the doctors would ask when their “last covered day” was — the last day Medicare, Medicaid, or their private insurance would pay for — and discharge them then, regardless of their condition.

“They have lives and jobs,” said Brandi George, a nurse who has worked at the hospital for three years, after beginning as a mental health technician. “And there’s no good medical reason for keeping them.” Because the nurses take extensive notes on patients' conditions, doctors would sometimes have to come up with new notes to explain the extended times, they said, such as a change in medication.

“If they’re on Medicare or Medicaid, they’ll milk it,” said Valerie Riling, who has worked at Brooke Glen for nearly two years. “We’re not doing right by our patients.”

“They’ll admit people just to fill a bed,” said George.