(CNN) -- Early speculation about the crash of Malaysia Airlines Flight 17 has centered on the possibility it could have been shot down as is passed over volatile eastern Ukraine.
After all, Ukrainian officials claim pro-Russia separatists, or possibly Russia itself, were behind the shoot downs of two Ukrainian aircraft in recent days.
A Ukrainian official also told CNN's Jim Sciutto Thursday that separatists claimed to have brought down another plane around the same time MH17 went missing.
But what could have shot down a commercial airliner traveling at 33,000 feet -- the altitude of the Malaysia flight as recorded by flight tracking site Flightaware.com?
Shoulder-fired missiles sometimes seen in the arsenals of rebel and separatists groups would be out: at best they can reach 15,000 feet, said CNN military analyst Rick Francona, a retired Air Force colonel.
"This would indicate a surface-to-air missile or an air-to-air missile and I think a surface-to-air missile is probably the best guess right now," he said.
One candidate is the Buk missile system operated by Russian and Ukrainian forces. A Ukrainian Interior Ministry adviser claimed this system was used to shoot down the airliner.
The missile system, known as the SA-11 in the United States, is operated by both Russian and Ukrainian forces, according to retired Brig. Gen. Kevin Ryan, the director of the Defense and Intelligence Project at the Belfer Center for Science and International Affairs.
And it's more than capable of taking down an airliner flying at that altitude, he said.
Such weapons travel with Russian troops at the division level, Francona said.
"So the Russians on the other side of the Ukrainian border will have all of this weaponry available to them," he said.
Other possibilities include Russian-made S-200 missiles that are operated by the Ukrainian military, as well as the Russian S-300 and S-400 missiles. The latter weapons are the Russian equivalent of U.S. Patriot missile defense batteries.
What seems unlikely is that pro-Russia separatists might have gained control of such a sophisticated piece of weaponry and used it to shoot down an airliner, Ryan said.
The Buk and other surface-to-air missiles are complex weapons that requires extensive training, sophisticated control systems, multiple vehicles and a team to operate.
"It takes a lot of training and a lot of coordination to fire one of these and hit something," he said.
That makes him think that if the plane really was shot down, it was taken down by a professional military force -- either on purpose or by accident.
"This is not the kind of weapon a couple of guys are going to pull out of a garage and fire," he said.
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