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.

UnitedHealth Group says it is cooperating with federal criminal and civil investigations involving its market-leading Medicare business.

The healthcare giant’s stocks slid on Thursday morning, extending a downward trend that began after the fatal shooting of UnitedHealthcare CEO Brian Thompson in December

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.

On Thursday, the giant health care conglomerate announced that it had “proactively reached out” to the Justice Department, after reports of a government investigation had surfaced.

UnitedHealth Group said Thursday it is facing Department of Justice investigations over its Medicare billing practices and is fully cooperating with it.

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.

UnitedHealth Group has said it is under investigation by the Department of Justice, which is probing its involvement in Medicare, a federal health insurance program for those over 65 and people with disabilities. Newsweek reached out to the Department of Justice for comment.

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.