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

The new civil fraud investigation is examining UnitedHealth's practices for recording diagnoses that trigger extra payments to its Medicare Advantage plans, including at physician groups the insurance giant owns, the report added. DoJ and UnitedHealth did not immediately respond to a Reuters request for comment.

UnitedHealth (UNH) stock was down Friday on a report from The Wall Street Journal that the health insurance company faces a Department of

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.