There was a text message from a passenger as well. No mention of terrorists, just that the pilots were passed out.
Sounds like faulty oxygen system in the cockpit. The crew oxygen system is in tanks, the passenger system is oxygen generators which are burned off a "candle" and provides the passenger with oxygen. The passengers probably used up all their emergency oxygen (its not very much), and froze to death, the pilots, unable to descend the plane, due to being passed out.