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 .
Senior U.S. official : Radar saw surface-to-air missile system track aircraft . A sophisticated surface-to-air missile was more likely used , experts say . Comment: The sophisticated weapon would require professional soldiers , expert says .