Reuters is reporting that the U.S. Justice Department has joined a whistleblower lawsuit against Minnetonka-based UnitedHealth Group that claims the country’s largest health insurer and its units and affiliates overcharged Medicare hundreds of millions of dollars, a law firm representing the whistleblower said on Thursday.

“We reject these more than five-year-old claims and will contest them vigorously,” UnitedHealth spokesman Matthew Burns said in a statement to Reuters.

The lawsuit, filed in 2011 and unsealed on Thursday, alleges UnitedHealth Group overcharged Medicare by claiming the federal health insurance program’s members nationwide were sicker than they were, according to the law firm Constantine Cannon LLP.

Shares lost 3.7 percent to $157.62.

The Justice Department has also joined in allegations against WellMed Medical Management Inc, a Texas-based health care company UnitedHealth bought in 2011.

The lawsuit by whistleblower Benjamin Poehling, a former UnitedHealth executive, has been kept under seal in federal court in Los Angeles while the Justice Department investigated the claims for the past five years. Constantine Cannon posted the lawsuit online when it was unsealed on Thursday.

No total damages were specified in the lawsuit.

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