It's early days, and much more information will be available once the flight information and cockpit voice recordings have been analysed.
Professor Michael Clarke, Director General of the Royal United Services Institute think-tank said: "Early reports said that [the aircraft] split into two and that suggests a catastrophic failure, not a mechanical failure, but that suggests perhaps an explosion on board. So I'd be much more inclined to think if we have to guess at this stage, it's much more likely to have been a bomb on board rather than a missile fired from the ground."
There are devices that trigger at a certain height. They got to 30,000 ft - said to be too high for ISIL weapons. I just hope this is not some kind of retaliation for the Ukarine crash. The aircraft did have a "tail impact" a while ago. Hopefully as the flight data recorders have been recovered and the wreck is available we should hear more fairly soon.
The flight tracking website Flightradar24 said that just before the radar signal was lost "the aircraft started to descend with a vertical speed of about 6,000ft (1,830m) per minute."
That in itself is an indication that there was something very wrong - an Airbus A321 would descend at the rate of between 1500-2000 feet per minute on a normal approach to landing, and would begin that descent at around 110 nautical miles from touchdown. I've been in an Airbus 321 which descended too slowly, or started its descent too late, and we ended up having to circle the airport to lose excess height.
Exact figures depend on other factors, like tailwind speed, but 6000 feet per minute is far too fast a rate under any kind of normal circumstances.
Technical difficulties might have been the reason for such a rapid descent - a big drop in cabin pressure, for instance, but you would expect the captain to radio Air traffic control in those circumstances. Apparently there was no contact from the aircraft after it left Sharm el-Sheikh.
Approximately 19 minutes after take-off the plane came down over the Sinai peninsula.
Whatever happened, it was sudden and catastrophic. These aircraft have a maximum takeoff weight of around 93 tons; they are extremely strong in terms of airframe and cabin construction. It would take a lot of force to break the cabin in two.
The airline involved has experienced one previous accident. In January 2011 a 27-year-old Tupolev Tu-154B caught fire on the runway at Surgut International Airport. Three passengers lost their lives.
The cause of the fire was an electrical arc produced when two generators not synchronised with each other were connected together instead of being connected to parallel busses.
I imagine it's possible that an explosion on board caused a rapid decompression, and the captain began to lose height as rapidly as he could although if the flight deck crew was able to operate the aircraft to that extent there should have been time for a Mayday transmission.
An explosion could have caused enough damage to prevent control via the fly-by-wire systems, and subsequent break up due to the stresses on an already damaged airframe resulting from a rapid descent.
It's all speculation, and the flight recorders should reveal a good deal of information. The flight-deck voice recorder is going to be of particular interest, I imagine. If there was an explosion, that device may have recorded any crew reaction - if there was time.