The probe is analyzing the company’s practice of frequently logging diagnoses that trigger larger payments to its Medicare Advantage plans, according to a report.

UnitedHealth Group shares tumbled early Friday on a report that the U.S. Department of Justice has started an investigation into the health care giant’s Medicare billing practices.

Eden Prairie-based health care giant says it’s not aware of the launch of any new Justice Department activity as reported by the Wall Street Journal.

UnitedHealth Group (UNH) is under fire over its Medicare billing practices. The Wall Street Journal reported Friday that the U.S. Department of Justice (DOJ) has launched a civil fraud investigation into how the company records diagnoses that lead to extra payments for its Medicare Advantage plans.

The Iowa Republican’s letter to the company cites findings from a series of Wall Street Journal articles.