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 cannot 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.