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

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.

UnitedHealth Group called The Wall Street Journal's report "misinformation" and said it was unaware of reported investigaiton.