UnitedHealth Group Inc. (NYSE: UNH) shares are trading lower Tuesday after the Wall Street Journal reported that Senator Chuck Grassley launched an inquiry into the company's Medicare billing practices.

The Dow Jones Industrial Average opened lower on Friday as a selloff in shares of UnitedHealth Group weighed on the blue-chip gauge. Earlier, the Wall Street Journal reported that the Justice Departme

UnitedHealth Group shares tumbled Friday on a report that the U.S. Department of Justice has started an investigation into the health care giant’s Medicare billing practices. The Wall Street Journal said federal officials have launched a civil fraud investigation into how the company records diagnoses that lead to extra payments for its Medicare

UnitedHealth Group saw its share price fall on Friday after the Wall Street Journal reported that it was facing a DOJ investigation.

The selloff came as more than $2.7 trillion worth of options expired, an event which typically spurs heightened volatility.

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.

A top Republican in the U.S. Senate is questioning Medicare Advantage billing practices at UnitedHealth Group following a critical report last year from a federal watchdog agency and a Wall Street Journal article last week on a new investigation by the Justice Department.