U.S. Senator Chuck Grassley requested detailed information on UnitedHealth's Medicare billing practices, in a letter to CEO Andrew Witty on Monday, amid reports suggesting the insurer may have leveraged billing rules to its advantage.

The Iowa Republican’s letter to the company cites findings from a series of Wall Street Journal articles.

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 U.S. Justice Department has launched an investigation into UnitedHealth's Medicare billing practices in recent months, the Wall Street Journal reported on Friday, citing people familiar with the matter.

The Justice Department is probing diagnoses and practices that triggered extra payments to UnitedHealth’s Medicare Advantage plans, people familiar with the matter say.

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