As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING TAXI OUT AND MOMENTS AFTER THE APU WAS SHUT DOWN, THE CABIN BEGAN TO FILL WITH SMOKE. AIRCRAFT RETURNED TO GATE AND PASSENGERS WERE DEPLANNED THROUGH THE FORWARD CABIN DOOR WITHOUT FURTHER INCIDENT. GROUND MAINTENANCE INSPECTED AND RAN THE PACKS USING BOTH ENGINES AND THE APU. NO SMOKE OR SMELL WAS NOTED. AIRCRAFT RETURNED TO SERVICE.