 The passengers, the report added, were also likely not wearing their oxygen masks, as there was no noise recorded on the ATC transmissions indicating they were attempting to assist the pilot.Investigators were not able to determine what caused the depressurisation of the aircraft, but stated likely causes included either an incorrect switch selection due to pilot error or a mechanical failure in the aircraft pressurisation system.The air traffic control recordings suggested it was unlikely a rapid decompression had occurred.