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 Friday on a report that the U.S. Department of Justice has started an investigation into the health care giant’s Medicare billing practices.The Wall Street Journal said federal officials have launched a civil fraud investigation into how the company records diagnoses that lead to extra payments for its Medicare Advantage,

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