Medicare practices sent the stock diving Thursday to a five-year low. The Justice Department is looking into potential Medicare fraud by the health insurer, according to a report by the Wall Street Journal,

Shares of UnitedHealth Group plunged more than 18% following a report that the Department of Justice is carrying out a criminal investigation into the health-care giant over possible Medicare fraud. The DOJ is reportedly focusing on the company's Medicare Advantage business 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.