The reported probe adds to a string of recent setbacks that have erased more than half, or over $300 billion, from its market value since November.

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,

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.