The Justice Department is probing diagnoses and practices that triggered extra payments to UnitedHealth’s Medicare Advantage plans, people familiar with the matter say.

The U.S. Justice Department is investigating UnitedHealth's Medicare billing practices, the Wall Street Journal reported on Friday, while the health insurer said it was unaware of any new probe underway.

The health insurance giant has called a new report from WSJ “misinformation,” but investors may not be so sure. Shares were down about 9% early Friday. UnitedHealth Group, America’s largest insurer, is facing yet another crisis.