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,

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.

The Justice Department is investigating UnitedHealth Group for possible criminal Medicare fraud, people familiar with the matter said. The healthcare-fraud unit of the Justice Department’s criminal division is overseeing the investigation,