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 has said it has not been notified by the Department of Justice of a reported criminal investigation. The Wall Street Journal reported on Wednesday that the firm is facing a DOJ investigation for possible Medicare fraud,

Shares of UnitedHealth Group tumbled 16% in intraday trading Thursday, leading S&P 500 decliners, on a report that the U.S. Department of Justice is investigating the embattled health insurance provider for possible criminal Medicare fraud.