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,

It keeps getting worse for UnitedHealth Group Inc. investors, as a report of a criminal investigation into its Medicare practices sent the stock diving again in early Thursday trading.

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.