The insurance and health care company UnitedHealth Group (NYSE: UNH) may be under federal criminal investigation for alleged Medicare fraud.

The U.S. Justice Department has not released the specific allegations against the company. However, they are reportedly related to UnitedHealth Group’s business practices within its Medicare Advantage plans, according to the Wall Street Journal, which first reported the news.

UnitedHealth Group quickly pushed back.

“We have not been notified by the Department of Justice of the supposed criminal investigation reported, without official attribution, in the Wall Street Journal today,” the company said in a statement. “The WSJ’s reporting is deeply irresponsible, as even it admits that the “exact nature of the potential criminal allegations is unclear. We stand by the integrity of our Medicare Advantage program.”

The investigation began last summer, the Journal reported. Following the report, UnitedHealth Group’s stock plummeted 15% as of Thursday afternoon.

The news comes two days after UnitedHealth Group CEO Andrew Witty stepped down for what the company said were “personal reasons,” in a statement. Chairman and President Stephen J. Hemsley has become the insurance and health care giant’s CEO, effective immediately.The company is the third largest hospice provider in the nation, primarily through its subsidiary Optum, according to research by Hospice News. This was achieved largely through the company’s 2023 acquisition of LHC Group for $5.4 billion.