(CNN) -- What kind of weapon can shoot down a fully loaded passenger jet flying at nearly 33,000 feet?
That could be a key question in the Malaysia Airlines Flight 17 crash investigation, as officials try to determine what was responsible for Thursday's plane crash in volatile eastern Ukraine.
A radar system saw a surface-to-air missile system turn on and track an aircraft right before the plane went down, a senior U.S. official told CNN's Barbara Starr. And a second system saw a heat signature at the time the airliner was hit. The United States is analyzing the trajectory of the missile to pinpoint where the attack came from, the official said.
Anton Gerashchenko, adviser to the Ukrainian Interior Ministry, said in a Facebook post that "terrorists" fired on the plane operating a Buk surface-to-air missile system.
A Ukrainian official also told CNN's Jim Sciutto on Thursday that separatists claimed to have brought down another plane around the same time Flight 17 went missing.
But what could have shot down the Malaysia Airlines flight?
Shoulder-fired missiles sometimes in the arsenals of rebel and separatists groups would be ruled out, experts said.
"At normal cruising altitude a civilian passenger aircraft would be out of the range of the sort of manned portable air (defense) systems that we have seen proliferate in rebel hands in east Ukraine," IHS Jane's Defence Weekly's Nick de Larrinaga said in an e-mail.
Such shoulder-mounted weapons at best can reach 15,000 feet, said CNN military analyst Rick Francona, a retired Air Force lieutenant 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, developed during the Soviet era and operated by Russian and Ukrainian forces.
The missile system, known as the SA-11 among NATO forces, is operated by both Russian and Ukrainian forces, according to retired Brig. Gen. Kevin Ryan, director of the Defense and Intelligence Project at the Belfer Center for Science and International Affairs at Harvard University.
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.
"It takes a lot of training and a lot of coordination to fire one of these and hit something," he said.
Typically, a surface-to-air battery missile consists of a command post vehicle, a radar vehicle, several self-propelled launchers, loader vehicles and even more vehicles to carry new missiles to the batteries as necessary, according to Dan Wasserbly, Americas editor for IHS Jane's.
Ryan concludes then that if the plane really was shot down, a professional military force -- either on purpose or by accident -- was responsible.
"This is not the kind of weapon a couple of guys are going to pull out of a garage and fire," he said.