After initially dismissing claims by Islamic State that the terrorist group brought down a Russian jet in the Sinai, Russian officials are now looking a lot closer at the idea that terrorism was involved in the crash.

An executive with the Russian airline that owned the plane is saying that "extrernal forces" were responsible for the crash.  Some experts are still discounting the possibility, given the altitude at which the plane was flying and the lack of evidence of a missile strike at the scene.

But the plane broke up in mid air very suddenly – so suddenly that the crew was unable to get off a mayday signal. 

CNN:

CNN aviation correspondent Richard Quest said it was "unusual" for an aircraft to go down roughly 20 minutes into a flight.

"At this point, a plane is on autopilot. It's reaching its initial cruising altitude, and there is little that can or should go wrong," he wrote in an analysis.

But the website Flightradar24 that tracks aircraft around the world said it had received data from the Russian planesuggesting sharp changes in altitude and a dramatic decrease in ground speed before the signal was lost.

"It's disturbing to me. It indicates me that something occurred possibly in the way of aerodynamic stall. I mean, an airplane just cannot fly at those lower speeds," said CNN aviation analyst Les Abend, although he cautioned that the Flightradar information was very preliminary.

Disintegration of the fuselage took place in the air, and the fragments are scattered around a large area" covering about 20 square kilometers (8 square miles), Viktor Sorochenko, executive director of Russia's Interstate Aviation Committee, told reporters Sunday.

Learning that the plane broke into pieces while in the air helps reduce the list of possible causes of the crash, but there are still plenty of scenarios, said CNN aviation analyst Peter Goelz.

"It narrows it down a little bit, but there are a number of issues that could have affected this plane," said Goelz, a former managing director of the U.S. National Transportation Safety Board (NTSB). "And terrorism has not been ruled out."

He suggested the disaster could have resulted from "some sort of catastrophic failure, perhaps caused by an earlier maintenance problem. It could have been a center fuel tank that might have exploded."

There had been tail damage to the plane previously, but the Russians insist it had been completely repaired.  The fact that the catastrophic damage apparently began with the tail separating from the fuselage could make the Russian airline executive's comments about "external forces" self-serving.  Covering up a failure in proper maintenance would not be surprising from a Russian company.

Islamic State claims about shooting down the jet are dubious at best.  But ISIS has captured all sorts of weapons in Syria and Iraq, including tanks, fighter jets, helicopters, and APCs.  No one really knows what weapons they have and whether they are capable of operating many of them.  Might they have gotten a hold of a sophisticated SAM system?  The possibility cannot be dismissed.

In truth, until experts unravel the information found in the black boxes and flight recorders, we simply won't know what brought the plane down.