The reported probe adds to a string of recent setbacks that have erased more than half, or over $300 billion, from its market value since November.

UnitedHealth Group (UNH) shares cratered more than 13% Thursday morning after The Wall Street Journal reported that the Department of Justice is conducting a criminal investigation into the company’s Medicare billing practices.

UnitedHealth Group shares fell nearly 6% in premarket trading on Thursday, after the Wall Street Journal reported that the U.S. Department of Justice was investigating the company for potential Medicare fraud.