What was found
AIR TURN BACK, AT APPROXIMATELY 1.0 PSI DIFFERENTIAL, A BANG WAS HEARD ACCOMPANIED BY A LOUD ROAR AND AN L-1 DOOR LIGHT. DEPRESSURIZED AIRCRAFT AND L1 DOOR LIGHT WENT OUT. VISUALLY INSPECTED DOOR LATCHES AND OPERATION OF L1 DOOR AND CONFIRMED DOOR WAS NOT SHUT PROPERLY. RE-SECURED DOOR SEAL RETAINER ON UPPER AFT CORNER OF DOOR AND REPLACED LOWER DOOR SEAL. PRESSURIZED AIRCRAFT CABIN AND LEAKED CHECKED GOOD. (M)