UnitedHealth Group revealed Thursday it is facing a Department of Justice investigation over its Medicare billing practices, adding to a string of setbacks for a company that owns the nation's largest and most powerful private insurer. In response at the time, the company said it stands "by the integrity of our Medicare Advantage program." That marked the second time this year that the insurer's Medicare Advantage business has come under federal scrutiny. The Journal also reported in February that the DOJ is conducting a civil investigation into whether the company inflated diagnoses to trigger extra payments to its Medicare Advantage plans. UnitedHealthcare's Medicare and retirement segment, which includes the Medicare Advantage business, is UnitedHealth Group's largest revenue driver, raking in $139 billion in sales last year.