The health insurance giant has called a new report from WSJ “misinformation,” but investors may not be so sure. Shares were down about 9% early Friday. UnitedHealth Group, America’s largest insurer, is facing yet another crisis.

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

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.