In a filing with the Securities and Exchange Commission, United Health Group said it has "full confidence" in its practices. It added that a court-appointed monitor found no wrongdoing after a decade-long probe into its Medicare Advantage business.

The health-care giant said it has started complying with both criminal and civil requests from federal investigators and it was cooperating with them.

UnitedHealth Group is scheduled to report second-quarter results on Tuesday morning, capping a tumultuous first half of the year for the health insurance giant.

The healthcare conglomerate said it was complying with requests from the agency after reaching out following media reports.

Stocks were mixed Thursday and the Dow Jones Industrial Average declined following slumps in Honeywell and IBM after the companies issued quarterly earnings updates and as UnitedHealth Group traded lower after saying it was “complying with formal criminal and civil requests” from the U.S. Department of Justice.

UnitedHealth shares tumbled Thursday after the health insurance giant confirmed that it’s the subject of a Department of Justice investigation. Monitor these key chart levels.

Mizuho's Jared Holz joins 'Fast Money' to talk the future of UnitedHealth after the stock slides on news of DOJ probe.

Shares of UnitedHealth Group fell sharply in premarket trading Thursday after the healthcare giant said it was “complying with formal criminal and civil requests” from the U.S. Department of Justice.