What was found
AT THE GATE PRIOR TO PUSHBACK, AIRCRAFT CABIN FILLED WITH SMOKE AND FLAMES WERE REPORTED COMING FROM APU EXHAUST DUCT BY GROUND PERSONNEL. NO FIRE INDICATION IN COCKPIT. APU WAS SHUTDOWN AND APU FIRE BOTTLE DISCHARGED. AIRCRAFT PASSENGERS WERE EVACUATED THROUGH FORWARD ENTRY DOOR VIA THE JETWAY AND AT DOOR 2L IN REAR CABIN AREA VIA EMERGENCY SLIDE. NO INJURIES WERE REPORTED. THE EMERGENCY SLIDE WAS REPLACED AND THE APU PLACED ON MEL.