One scary possiibility is some terrorist group was able to
jury rig a launch system for the Soviet-era 3M33 (SA-8
Gecko) surface-to-air missile, including the guidance radar system. Unlike a MANPAD missile, the 3M33 missile can hit a target up to 39,000 feet in height by radar guidance--well within the flight envelope of the 777-200ER at cruising altitude.
Indeed, one wonders was TWA Flight 800 shot down with such a jury-rigged missile launch system?