UnitedHealth Group announced Thursday that it is under an investigation by the Department of Justice over the insurance company's Medicare billing practices.

UnitedHealth says it proactively reached out to the DOJ following a February report from the Wall Street Journal that first indicated the DOJ was launching a probe. The company says it is fully complying with the investigation.

"The Company has full confidence in its practices and is committed to working cooperatively with the Department throughout this process," UnitedHealth wrote in a filing with the Securities and Exchange Commission (SEC).

"The Company has a long record of responsible conduct and effective compliance. Independent CMS audits confirm that the Company’s practices are among the most accurate in the industry," the filing continued.

UnitedHealth's stock slid 15% in the fallout of the Journal's initial report on the criminal probe in February.

The new probe only added to the health insurer's list of problems, including multiple government inquiries, a sudden change in top leadership and a pulled outlook in the face of soaring medical costs.

The leadership switch involves CEO Andrew Witty's abrupt departure and the withdrawal of its 2025 forecast, which triggered an 18% drop in shares to a four-year low in early May.

In February, the WSJ revealed a civil fraud investigation into UnitedHealth's Medicare practices, while Sen. Chuck Grassley, R-Iowa, launched an inquiry into the company's billing methods, demanding detailed compliance records.

For decades, the company has flourished by leveraging its dominance in insurance and growth in the Medicare market, the U.S. government program that covers medical costs for the elderly.

UnitedHealth is not alone in taking fire, however, as the health insurance industry as a whole has come under government scrutiny after a May lawsuit accused three major U.S. insurers of paying kickbacks to brokers to steer patients into their plans.

Reuters contributed to this report