Question. What if malefactor ...?
I've been pondering similar for the last few days.
An explosive or mechanical device wasn't even needed. Just access to the aircraft for someone with a couple of tools and a basic knowledge of flight dynamics (and/or directed by someone who had more advanced knowledge). The aircraft was on the ground for 12 hours, possibly unsecured. Various acts* of mechanical sabotage could have taken place in that time.
In my eyes, sabotage undetectable to the naked eye or sniffer dogs could be far more dangerous than explosives.
However,
why did nothing happen until that particular altitude? If the airframe was seriously compromised, wouldn't something have happened/been detected earlier, even just an erroneous sensor reading or vibration? Would it have even left the ground?
*public forum - unsure whether we want to discuss exact details here?