UnitedHealth Group, America’s largest insurer, is facing yet another crisis. The company’s stock price (NYSE: UNH) plummeted in early trading this morning after a new report alleged that the private insurance provider is facing a probe from the U.S. Department of Justice (DOJ) over its Medicare billing practices, a claim UnitedHealth Group denies. Here’s what to know:

Shares in UnitedHealth Group fell this morning after the Wall Street Journal published a report stating that the $400 billion company was under investigation by the DOJ over its Medicare billing practices. The report states that the investigation is a civil fraud case and not a criminal one.

At the heart of the investigation are DOJ concerns that UnitedHealth is recording patient diagnoses that generate additional payments to its Medicare Advantage plans.

As the WSJ notes, UnitedHealth Group (aka UHG) and other insurers get lump sum payments from the federal government via the Medicare Advantage system. If patients have certain conditions, those lump sum payments can increase, generating more profit for insurers.

Fast Company reached out to the DOJ for comment.

The alleged investigation follows a

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