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 investigation is evaluating Medicare billing protocols, according to the report.UnitedHealth shares sank after The Wall Street Journal reported that the insurer's Medicare billing protocols are under Justice Department investigation.