Satellite images show a plume of smoke left by the ground-to-air missile that brought down Malaysia Airlines flight MH17. Infrared sensors recorded the moment when the airliner exploded.
As US analysts sift through fragments of intelligence to try to pin down who fired the missile and why, and where it came from, they are running into difficult questions.
The US assessment, outlined by Ambassador Samantha Power at the UN Security Council, is that the airliner was "likely downed by a surface-to-air missile, an SA-11, operated from a separatist-held location in eastern Ukraine".
US officials said authorities were trying to determine who fired the Russian-made missile, whether Russian operatives aided in the attack and how the weapon arrived on territory held by Ukrainian rebels backed by Moscow.
Those determinations could be key to any international response to the downing of the plane that killed all 298 people aboard and sharply raised the stakes in Ukraine's conflict.
The American analysts have based some of their conclusions so far on technical data from advanced spy satellites whose principal use is to provide early warning of intercontinental ballistic missile launches.
PLUME OF SMOKE
The satellite data included an image of a plume of smoke left in the missile's trail that allowed analysts to calculate a launch area near the Russia-Ukraine border which is dominated by pro-Russian separatist fighters, officials said.
It also included data culled from infrared sensors, which detected the explosion of the jet, they said.
Although the possible launch area extends to both sides of the border, the most likely location is in rebel-held territory close to where the wreckage of the plane plummeted from the sky, US officials said.
The Pentagon, White House and UN Ambassador Samantha Power all publicly suggested that because the SA-11 was a complicated weapon, whoever fired it may have had to have had Russian training or help in order to operate it.
"It strains credulity to think that (the missile) could be used by separatists without at least some measure of Russian support and technical assistance," Pentagon spokesman Rear Admiral John Kirby told reporters.
But US officials stopped short of accusing Russia fighters of playing a direct role in launching the missile. No determination had yet been made as to "who pushed the button", one official said.
The missile may have been launched by either a pro-Russian Ukrainian separatist or a Russian national, the officials said.
They said they did not believe the missile was launched by a pro-government Ukrainian and were working on the assumption that it was fired by either a pro-Russian Ukrainian separatist or a Russian national.
LIMITS OF INTELLIGENCE
In an indication of the limitations of US intelligence capabilities, officials said they were unsure how the missile arrived in the launch area.
There was no US intelligence showing an SA-11 crossing the border into Ukraine, the Pentagon said.
Power said that separatists had been spotted hours before the incident with an SA-11 system "at a location close to the site where the plane came down". She said a Western reporter had indicated early on Thursday that an SA-11 system was reported near Snizhne, in eastern Ukraine.
Power told the Security Council the United States was not aware of any similar Ukrainian-controlled missile systems in the area. "Since the beginning of the crisis, Ukrainian air defences have not fired a single missile," she said.
An official said the satellites used to detect the missile, known as Defense Support Program (or DSP) satellites, orbit the Earth about 36,000 kilometres over the equator, and are operated from a control station at Buckley Air Force base in Colorado.