The Wall Street Journal first reported the civil fraud investigation into whether the insurance giant’s practices related to patient diagnoses force higher payments from the government’s Medicare Advantage program.

Shares of UnitedHealth fell 9.3% at 10:06 a.m. in New York. Humana Inc., which has a large Medicare business, was down 5.5%.

The DOJ declined to comment on the news. UnitedHealth pushed back against the report. 

“We are not aware of the ‘launch’ of any ‘new’ activity,” the insurer said in an emailed statement. “Any suggestion that our practices are fraudulent is outrageous and false.”

Meanwhile, the Justice Department has been conducting a broad antitrust investigation of UnitedHealth’s practices that started under the Biden administration. That probe emerged out of concerns about UnitedHealth’s acquisitions of health-care providers and data companies, Bloomberg reported at the time. Decisions about that inquiry will fall to Trump’s antitrust chief Gail Slater, who is awaiting confirmation by the Senate.

It wasn’t immediately clear whether the Medicare billing inquiries are related to the antitrust investigation.

The media could not be loaded, either because the server or network failed or because the format is not supported.

More bad news

Morningstar analyst Julie Utterback said UnitedHealth’s dramatic share decline looks like an “overreaction” given the relatively small size of the company’s Medicare Advantage division compared to its overall business. 

“But when you look at what’s happening post-shooting and in the DOGE era, regulators may be more emboldened than usual to take action on companies like this that perhaps are milking the system, if you will,” Utterback said.

The government has been scrutinizing costs in Medicare Advantage, a privately administered version of Medicare, a federal insurance program for elderly. A key group that advises Congress on Medicare policy has raised concerns about how the program’s payment system can be manipulated, declaring an “urgent need for a major overhaul,” in a 2024 report. 

More than half of those on Medicare now get their benefits through Medicare Advantage, private plans which get payments from the US government. Each year, the plans submit giant data files to Medicare with diagnostic codes meant to reflect their members’ illnesses. Those codes determine how much the insurers get paid.

Typically, insurers get paid more for patients with more severe illnesses.

In recent years, the Biden administration attempted to constrain payments to Medicare Advantage plans, which lawmakers, watchdogs and whistleblowers have accused of wasting tax dollars.

UnitedHealth has been under intense scrutiny in recent months. In December, the head of its insurance division was killed on his way into an investor conference. The shooting led to widespread public outcry from Americans about how frequently insurers deny medical care. 

Earlier this month, the insurer’s shares slipped after billionaire investor Bill Ackman, in a now-deleted post on X, suggested the company overstated profits after a doctor said she had to interrupt medical care to deal with a call from the insurer. UnitedHealth has since responded that hospital error led to the phone call to the surgeon and said that it contacted the US Securities and Exchange Commission with concerns about Ackman’s deleted post.

Read more from Fortune