One of the big questions is why did the pilots not call if there was an emergency.  Here is a viable scenario.  When the plane dropped to 12,000 feet, it was too late.  All the passengers had passed.  The pilot and co=pilot were almost gone as well.  but they did not desire to report that the passengers had all passed so they set auto pilot for out to sea where no one would ever discover what had happened.  They simply did not want to live with all their passengers dead.  Not a pretty thought.  .