It extends a tumultuous period for UnitedHealth marked by the killing of a top executive, a costly cyberattack against its subsidiary and high medical costs.

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 Wall Street Journal had previously reported that Medicare paid UnitedHealth billions of dollars for questionable diagnoses.