If the drones are there and they can reach that height, how can it be ruled out as a possibility? And if the US is vetting those who work with the drones, mygosh, ISIS sympathizers were probably at the front of the line.
Because we all know that ISIS is a bunch of techie nerds that can hijack a drone with a missle and fly it to 30000 ft and hit a target moving at 500 mph ... and that is so much easier than getting a sympathetic baggage handler to stow a bomb with an altimeter in a suitcase on board.