UnitedHealth Group is being investigated by the Justice Department for possible Medicare Fraud, according to a report. The healthcare-fraud unit of the Justice Department’s criminal division is overseeing the investigation, which has been ongoing since last summer, sources told The Wall Street Journal.

UnitedHealth Group stock fell early 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 citing high medical costs in its insurance division.

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.