The investigation is the latest worry for UnitedHealth, which is already facing an antitrust probe and regulatory pushback over its planned $3.3 billion acquisition of Amedisys. UnitedHealth has been pushed to the forefront of the heated debate over soaring healthcare costs and patient care.

The stock is having its worst day in five years, even as the company denied any new Justice Department investigation and a Mizuho analyst recommended buying.

Key Takeaways U.S. equities dropped at midday following the latest data on consumer sentiment and a report that UnitedHealth Group is under a federal probe. UnitedHealth Group is said to be facing a Department of Justice probe of its Medicare billing practices.