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,

UnitedHealth Group shares tumbled early Friday on a report that the U.S. Department of Justice has started an investigation into the health care giant’s Medicare billing practices.