The unthinkable
I have a horrible feeling, and I hope I am wrong, but looking at all the facts I can't help but suspect the involvement of one of the flight crew.
I am a retired investigator, (not in the aircraft industry) and i would have had the pilots and crews houses searched the same day. Furthermore I would be instigating investigations into the background of every other person on the plane and the details of the senders of every item of cargo.