The stupid reporters and such meant to say "contaminated oxygen" not "poison gas."
Not nearly enough info to even start to guess, even wild guesses at this time would be inappropriate.
That phone message bothers me. How did he know the pilots were unconsious? The doors would have been secure, yes?? I'd check for another identity for this dude who said goodby. He was pretty cool about it. Wonder who else was on the plane? Could the steward/stewardess have given something to the pilots to drink?