Shares of UnitedHealth Group (NYSE: UNH) are falling on Thursday. The collapse comes as the S&P 500 gained 0.1% and the Nasdaq Composite fell 0.4%. The troubled health insurance giant is facing reports of a Department of Justice (DOJ) investigation into possible Medicare fraud.

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UnitedHealth Group is being investigated by the Justice Department for possible criminal Medicare fraud, The Wall Street Journal reported, citing people familiar with the matter. The healthcare-fraud unit of the Justice Department's criminal division is overseeing the investigation,

UnitedHealth Group announced that CEO Andrew Witty is stepping down due to 'personal reasons,' and former CEO Stephen Hemsley will replace him.

Stephen Hemsley hasn’t been CEO at UnitedHealth Group in nearly a decade yet he’s never really left. Hemsley, 72, has remained chairman and a major shareholder in the health care behemoth — and now is back in the CEO suite and is expected to reinstate his brand of operational and financial discipline that turbocharged the company’s growth.

UnitedHealth Group CEO Andrew Witty is stepping down from his post for personal reasons and the health care giant says it’s suspending its full-year forecast due to higher-than-expected medical costs.