Paris. The CVR recording enabled investigators to identify Prodromou as the flight attendant who entered the cockpit in order to try to save the plane. He called "mayday" five times, but because the radio was still tuned to Larnaca and not Athens, he was not heard by ATC. His voice was recognized by colleagues who listened to the CVR recording. Many of the bodies recovered were burned beyond recognition by the post-impact fire. Autopsies on the crash victims showed that all were alive at the time of impact, but it could not be determined whether they were conscious as well. The emergency oxygen supply in the passenger cabin of this model of Boeing 737 is provided by chemical generators that provide enough oxygen, through breathing masks, to sustain consciousness for about 12 minutes, normally sufficient for an emergency descent to 10,000 feet (3,000 m), where atmospheric pressure is sufficient for humans to sustain consciousness without supplemental oxygen. Cabin crew have access to portable oxygen sets with considerably longer duration. The Hellenic Air Accident Investigation and Aviation Safety Board (AAIASB) listed the direct causal chain of events that led to the accident as: non-recognition by the pilots that the pressurization system was set to "manual", non-identification by the crew of the true nature of the problem, incapacitation of the crew due to hypoxia, eventual fuel starvation, impact with the ground. Previous pressurization problems On 16 December 2004, during an earlier flight from Warsaw, the same aircraft experienced a rapid loss of cabin pressure and the crew made an emergency descent. The cabin crew reported to the captain that there had been a bang from the aft service door, and that there was a hand-sized hole in the door's seal. The Air Accident and Incident Investigation Board (AAIIB) of Cyprus could not conclusively determine the causes of the incident, but indicated two possibilities: an electrical malfunction causing the opening of the outflow valve, or the inadvertent opening of the aft service door. The mother of the first officer killed in this crash claimed that her son had repeatedly complained to the captain about the aircraft getting cold. Passengers also reported problems with air conditioning on Helios flights. During the 10 weeks before the crash, the aircraft's environmental control system was repaired or inspected seven times. A 2003 flight of a Boeing 737 between Marseille Airport and Gatwick Airport showed that a cabin-wide