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 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.

UnitedHealth Group shares tumbled early Friday on a report that the U.S. Department of Justice has started an investigation into the health care giant’s Medicare billing practices.