I find it hard to believe that you could make a bomb sufficient to bring down an airliner from materials beyond security.
i agree with the limited knowledge at my disposal that making a bomb in that situation is rather difficult, but i wouldn't go so far as to say it's impossible. nor would it be the only way to down an aircraft. remember, boxcutters, false threats and fanaticism was all that was needed to create 9/11.
rather, it's a matter of taking control of the aircraft.
you should look no further than our prisons. the long-time inmates in there are quite clever and resourceful. they would have a field day making weapons out of all the stuff that's available to them beyond the check points at the airports. i assume terrorists may posses the same acumen.
i don't mean to cause anyone unecessary alarm, but if inmates in our prisons can make deadly weapons out of newspapers and toothbrushes, then the possibilites are endless as to what terrorists can cobble together on the concourses and lobbies for the purpose of terrorizing and overpowering passengers and crew and creating more manned missiles of mass destruction.
to me, more than anything else, if all passengers make an effort to be situationally aware of circumstances that might expose a direct threat to all on board, and use discretion in alerting the proper authorities, it would go a long way toward providing safe passage on mass transit venues.