In a filing with the Securities and Exchange Commission, UnitedHealth 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 on Thursday publicly confirmed it was under investigation by the U.S. Department of Justice and said it was complying with both criminal and civil requests from the federal agency.

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.

Once considered a reliable bet, the health care behemoth has lost its appeal following a series of management missteps. UnitedHealth’s stock has fallen more than 40% so far this year. Independent CMS audits confirm that UnitedHealth’s practices are among the most accurate in the industry, the company said.

We recently compiled a list of 10 Most Undervalued Healthcare Stocks to Buy According to Analysts. It is the largest U.S. health insurer, operating through its insurance arm (UnitedHealthcare) and health services division (Optum).

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.

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