U.S. stocks drop at Friday's open and are on track for a weekly loss. The DOJ's probing UnitedHealth, sending shares and indexes lower.

Key Takeaways Shares of UnitedHealth Group sank Friday following a report that the U.S. Department of Justice is investigating the company's diagnosing practices.The Wall Street Journal reported that the DOJ is probing whether the company tends to over-diagnose conditions that can earn larger Medicare Advantage payouts.

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 Department of Justice has reportedly launched an investigation into UnitedHealth Group's Medicare billing practices, adding to the scrutiny the company is facing.

Any suggestion that our practices are fraudulent is outrageous and false," the insurer said.