What was found
APPROXIMATELY FIVE MINUTES INTO GROUND RUN IN PREPARATION FOR A FLIGHT, APU EXHIBITED A LOUD SCREECHING OR GRINDING NOISE FOLLOWED BY TWO LOUD "POPS". HEAVY OIL SMOKE IMMEDIATELY BEGAN EXITING CABIN AND COCKPIT AIR CONDITIONING DUCTING. APU SHUTDOWN WAS INITIATED. APU FIRE EXITINGUISHING SYSTEM DISCHARGED. DETERMINED THAT NO FIRE HAD OCCURRED AND THAT APU FLUIDS WERE CONTAINED WITHIN THE APU, VENTED AND EXHAUST SYSTEMS. GROUND OBSERVERS REPORTED TWO LARGE FIREBALLS EXITING THE APU EXHAUST, AND EXCCESSIVE AMOUNTS OF SMOKE EXTERNAL OF THE ACFT EMPENNAGE. INITIAL BORESCOPE INVESTIGATION REVEALED THE APU SUFFERED A "CATASTROPHIC INTERNAL FAILURE". NO DETAILS AS TO THE CAUSE ARE AVAILABLE AT THIS TIME.