Hi, it’s John in New York. I’ve spent years following a story about how whistleblowers alleged systematic fraud at private Medicare Advantage plans. A reported investigation at one of the biggest companies in that business jolted investors last week.

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 Chief Executive Andrew Witty a letter demanding detailed information on the company’s Medicare billing practices Monday.