UnitedHealth Group stock fell Thursday following a report that the Justice Department is investigating the company for possible criminal Medicare fraud. The news comes only days after the struggling health insurer announced the departure of its CEO Andrew Witty due to personal reasons and suspended its full-year guidance,

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.