UnitedHealth Group (NYSE:UNH) is under investigation by the Department of Justice for potential Medicare fraud. The probe is centered around the insurer's Medicare Advantage business, The Wall Street Journal reported.

KEY TAKEAWAYSShares 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.