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.

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 (UNH) stock crashed on Friday morning in light of a new United States (US) Department of Justice (DOJ) probe into possible overcharging of the federal government entity for retiree health services. UNH stock plunged as much as 12.7% at the open, but shares are now down about 9% closer to lunchtime near $457.