The Veterans Affairs Department is investigating whether Acadia Healthcare, one of the country's largest chains of psychiatric hospitals, is defrauding government health insurance programs by holding patients longer than is medically necessary, according to three people with knowledge of the inquiry.

The investigation, led by the agency's inspector general, comes three weeks after Acadia told investors that it was facing scrutiny for its admissions practices from several other federal investigators, including prosecutors in Manhattan and a grand jury in Missouri. The company, which relies on government insurance programs like Medicare and Medicaid for much of its revenue, said it was also expecting to receive inquiries from the Securities and Exchange Commission and other agencies.

Acadia told investors that it was "fully cooperating with authorities and, at this time, cannot speculate on whether the outcome of these investigations will have any impact on its business or operations." The company has denied claims that it was improperly holding patients and has said that all decisions about care are made by licensed medical professionals.

The Veterans Affairs Department did not immediately respond to questions about the new investigation.

The New York Times reported in September that Acadia was holding patients against their will in ways that appeared to violate state laws. The Times reported that some patients arrived at emergency rooms seeking routine mental health care but then were sent to Acadia facilities and locked in. The company's employees often held patients until their insurance coverage ran out, even when there was no medical justification.

The Times article was based on official complaints, court records and interviews with patients, as well as more than 50 current and former Acadia employees.

Last month, Acadia agreed to pay almost $20 million to settle Justice Department claims that the company had defrauded government health insurers by holding patients longer than medically necessary and admitting people who didn't need to be there. Acadia did not acknowledge wrongdoing in that case, which covered conduct from 2014 through 2017.

The fresh round of investigations are focused on recent events. The veterans agency is looking into whether Acadia billed insurance programs for patients who were stable enough to be released and did not need intensive inpatient care, according to two of the people with knowledge of the investigation, who requested anonymity because it has not been made public.

Several former Acadia employees in Georgia and Missouri have also recently been interviewed by agents from the F.B.I. and the inspector general's office of the Health and Human Services Department.