As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BEFORE PUSH BACK, GROUND MAINTENANCE REPORTED FIRE AND SMOKE & INSTANT FLAME FOR A DURATION OF 1 SECOND TOGETHER WITH SURGE NOISE. NO FIRE WARNING MESSAGE, FLIGHT CREW CONFIRM WITH GROUND MAINTENANCE, USED THE APU FIRE EXTINGGUISER BOTTLE. CHECKED APU COMPARTMENT FOUND OIL LEAK. PERFORM VISUAL CHECK NO STRUCTURAL DAMAGE AND NO TRACE OF FIRE.