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.

The U.S. Justice Department has launched an investigation into UnitedHealth's Medicare billing practices in recent months, the Wall Street Journal reported on Friday, citing people familiar with the matter.

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