Tests on victims of a Cyprus airliner crash showed no sign of carbon monoxide poisoning, a finding which one aviation medicine expert said on Friday could indicate the 121 passengers and crew died from lack of oxygen. Helios Airways, the operator of the Boeing 737 which crashed near Athens last Sunday, said it would send its other Boeings to Sweden for safety checks after an incident involving another of its aircraft on Friday. Investigators are struggling to discover why the pilot, co-pilot and many passengers on the jet apparently fell unconscious before the plane crashed into a mountainside, killing all six crew and 115 passengers. One theory was that the worst disaster in Greek and Cypriot aviation history was caused by faulty air conditioning or a fire releasing poisonous carbon monoxide fumes. But the absence of carbon monoxide appeared to rule this out. "We are still doing tests for other gases, poisons, drugs and alcohol," Greece's Chief Coroner Philippos Koutsaftis said after meeting Justice Minister Anastasios Papaligouras. Of six victims examined, five, including the co-pilot, showed no signs of breathing in carbon monoxide, while a stewardess had a minimum level of seven percent which was not considered dangerous, Koutsaftis told reporters. --more 2327 Local Time 2027 GMT