Washington ( CNN ) -- A classified U.S. intelligence analysis says it is most likely that pro-Russian separatists in eastern Ukraine fired the missile that downed Malaysia Airlines Flight 17 , CNN has learned . A U.S. defense official with direct access to the latest information detailed the analysis to CNN but declined to be identified because of the sensitivity of the information . The preliminary analysis concludes the missile was from a Russian-made Buk system , known as the SA-11 among NATO forces , the official said . U.S. military intelligence analysts concluded only the SA-11 or the SA-20 has the warhead , range and ability to shoot down an airliner , the official said . The Ukrainians are not believed to have an SA-20 system , and there is no intelligence suggesting Russia ever transferred one across the border . However , U.S. intelligence has concluded that pro-Russian separatists have a Buk system . The official says it 's unclear if the separatists captured it from Ukraine or Russia transferred it to them . The U.S. military and intelligence community expect to have a more definitive assessment of what happened by the end of the weekend . `` We want to put together a solid picture '' on each element of the event , the official said . That assessment is expected to remain classified for now , but will be shared among NATO nations , the official said . It may eventually be shared publicly to show what the United States believes happened . `` We can not say with complete certainty , but we do not think the missile came from the Russian side of the border , '' the official said . `` All the indications are it was fired from inside eastern Ukraine . '' The initial assessment also suggests separatists thought they were shooting at a Ukrainian military transport plane . Air Force Gen. Philip Breedlove , NATO 's top military commander , who also is head of the U.S. European Command , is leading much of the U.S. military 's technical intelligence effort looking at all satellite , radar and other intelligence data from the area . He is coordinating with other NATO members and east European nations in the region scouring their radar and intelligence assets to see if there is specific data showing the event . For now , the United States has some data that shows a missile launch system was turned on and operating in the area at the time of the plane 's downing as well as separate data that shows a heat signature of a major event , believed to be the plane being shot down . MAPS : What route did the flight take ?
Preliminary analysis concludes missile was from a Russian-made system , official says . U.S. intelligence has concluded that pro-Russian separatists have a Buk system , official says . `` All the indications are it was fired from inside eastern Ukraine , '' official says .