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

The health care giant said Thursday that it had contacted the Department of Justice after reviewing media reports about investigations into certain elements of its business.

“(UnitedHealth) has a long record of responsible conduct and effective compliance,” the company said in a Securities and Exchange Commission filing.

Earlier this year, The Wall Street Journal said federal officials had launched a civil fraud investigation into how the company records diagnoses that lead to extra payments for its Medicare Advantage, or MA, plans. Those are privately run versions of the government’s Medicare coverage program mostly for people ages 65 and over.

The company’s UnitedHealthcare business covers more than 8 million people as the nation’s largest provider of Medicare Advantage plans. The........

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