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If it had been a hypoxia event the aircraft wouldn't have turned around, turned off the transponder unless it was a coincidental failure. The fact that the engines were still reporting engine data, if that is true, is certainly an indication of "not" a cataclysmic event. I spent many a night over my career flying the Narita to Singapore and Bangkok runs, plus a few operational flights in the Navy "way back when" in that area. The radar coverage is minimal but once you get in close to overflying land, there should have been at least a secondary target picked up on radar somewhere. Given the fuel load, with reserves, it still had some legs left if it did remain airborne. (Pakistan?) My fear is that it did indeed land in one piece, the passengers "accommodated" that the aircraft might be used for a deadlier purpose in the future. I am not a conspiratist by any means, but given the extremes the terrorist groups have gone to in the past...who knows?
I hope I'm wrong as well as hope for some closure soon to those who don't know what has happened to their loved ones.