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

The Wall Street Journal says the U.S. Justice Department in recent months has launched an investigation into UnitedHealth Group’s billing practices to see if the company fraudulently used diagnosis data to trigger extra Medicare Advantage payments.

The health insurance giant has called a new report from WSJ “misinformation,” but investors may not be so sure. Shares were down about 9% early Friday. UnitedHealth Group, America’s largest insurer, is facing yet another crisis.