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 Justice Department is probing diagnoses and practices that triggered extra payments to UnitedHealth’s Medicare Advantage plans, people familiar with the matter say.

UnitedHealth Group (UNH) shares declined more than 11% in premarket trading after The Wall Street Journal reported that the Justice Department has launched a civil fraud investigation into the company’s Medicare billing practices.