** Shares of healthcare conglomerate UnitedHealth UNH.N fall 8.7% to $458.7 premarket
** U.S. Department of Justice launched investigation into co's Medicare billing practices in recent months, Wall Street Journal reports, citing people familiar with the matter
** DOJ's new civil fraud investigation is examining UNH's practices for recording diagnoses that trigger extra payments to its Medicare Advantage plans, according to the newspaper
** Stock has fallen ~5% in last 12 months
(Reporting by Padmanabhan Ananthan)
((Padmanabhan.Ananthan@thomsonreuters.com;))