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

Sen. Chuck Grassley, chairman of the Senate Judiciary Committee, sent UnitedHealth Group CEO Andrew Witty a letter demanding detailed information on the company’s Medicare billing practices Monday. The letter—which cited findings from a series of Wall Street Journal articles published over the last year—said “the apparent fraud,