Evidence gathered so far suggests separatists launched the SA-11 surface-to-air missile that blew up the Malaysian airliner on July 17, but it remains unclear "who pulled the trigger" and why, said a senior intelligence official who spoke on condition of anonymity.
"The most plausible explanation... was that it was a mistake," and that the missile was fired by "an ill-trained crew" using a system that requires some skill and training, the official said.
Read: MH17 black boxes to be sent to UK lab: Malaysia
The intelligence official cited previous incidents over the years in which both Russian and US forces have mistakenly shot down civilian airliners.
A Korean airliner was downed by a Soviet fighter jet in 1983 and US naval forces mistakenly shot down an Iranian civilian passenger plane in 1988.
"We've all seen mistakes in the past," the official told reporters.
US satellite and other "technical" intelligence confirmed the airliner with 298 people on board was hit by an SA-11 surface-to-air missile from an area controlled by the pro-Russian rebels.
"It's a solid case that it's a SA-11 that was fired from eastern Ukraine under conditions the Russians helped create," the senior official said.
"There are two things we don't know... who exactly pulled the trigger. We don't have a name, a rank or a nationality," the official said.
"And we also don't know why."
It appeared those who fired the SA-11 missile were relying on a lone radar that is part of a missile battery and not a larger network of radar that would give a more complete picture of air traffic, officials said.
The SA-11 is designed to be used in an "integrated air defense system" but with only one narrow radar beam, the missile launchers would have "a much more fuzzy picture," said a second intelligence official.
Read: MH17 has strong echoes of 1983 Korean tragedy
Russian paramilitary operatives have been spotted on the ground in eastern Ukraine but US spy agencies had no explicit proof that Russians were with the SA-11 unit that fired on the airliner, officials said.
Although the United States had observed a flow of heavy weapons, including air defense systems, into Ukraine from Russia, intelligence agencies had not seen the larger SA-11 missiles being moved into the country before the airliner was downed, officials said.
The Russian military has been training the rebels at a large base in Rostov on various weapons, including air defense systems, officials said.
But it was unclear if the Russians trained the separatists on the SA-11 missile batteries, officials said.
The senior intelligence officials said they chose to brief reporters partly to counter what they called "misinformation" from Russia and its state-controlled media over the incident.
Russia and its proxies were staging a "full court press" to spread misleading propaganda about the downing of the airliner, the official added.
Allegations that the Malaysian Boeing 777 took evasive action in the air, similar to how a military plane might manuever, had no basis and the reports amounted to "a classic case of blaming the victims," the senior official said.
The claim that the Ukrainian government had shot down the plane was not realistic, as Kiev had no such missile systems in that area, which is clearly under the control of the rebels.
That scenario would mean Ukrainian government troops would have had to fight their way into the area, fire at the passenger plane and fight their way out again, the official said.
Moreover, the Ukrainian government would have had to orchestrate claims by separatists on social media that they had shot down a plane.
"That is not a plausible scenario to me," the official said.
The Ukrainian government had no reason to be deploying anti-aircraft weapons, as the separatists were not flying helicopters or staging bombing raids from the air, officials said.
But the separatists had a motive to use surface-to-air missiles, as they faced offensives by Ukrainian government troops relying on helicopters and cargo planes, they said.
A Russian general has cast doubt on the US government's account, demanding Washington produce satellite images as proof.
But US General William Shelton, head of US Air Force Space Command, said the Russian officer's comments sounded like "an act of desperation."
US military satellites produced round-the-clock global coverage, he said. "They are very sensitive, and they are very accurate," Shelton told reporters at a Pentagon briefing. He declined to say if the system could pick up an SA-11 launch.
MH17 black boxes to be sent to UK lab
Meanwhile, the black boxes recovered from the crash of Malaysia Airlines flight MH17 in eastern Ukraine will be sent to British aviation investigators for analysis, Malaysia has said.
The decision by the Dutch-led team probing the disaster came after pro-Russian rebels controlling the crash site handed the boxes over to Malaysian officials on Tuesday, following an intense international outcry.
Transport minister Liow Tiong Lai said in a statement late Tuesday that it was normal procedure to send the boxes, which record cockpit activity and flight data, to the nearest laboratory approved by the United Nations aviation agency, the International Civil Aviation Organisation (ICAO).
"The international investigation team, led by the Netherlands, has decided to pass the black boxes to the UK Air Accidents Investigation Branch for forensic analysis," he said.
The boxes will be flown to Farnborough, England, accompanied by Malaysian officials and other members of the investigation team, Liow added.