Last Updated Jul 18, 2014 9:30 PM EDT
LONDON -- To figure out why a Malaysian jetliner fell from the sky, investigators will use the wreckage of any missile found to determine where it came from and who fired it, experts said Friday. That may be easier said than done in the middle of a war zone.
The first international monitors to arrive on the scene, 24 hours after Malaysia Airlines Flight 17 came down, found bodies strewn on the ground and restrictions from armed militiamen.
That gives a sense of the formidable obstacles investigators face in deciphering a disaster scene spread over eight square miles of contested ground in eastern Ukraine - amid a conflict in which both sides have interests that may outweigh a desire to uncover the truth.
"We are in a country that is at war, and that is in a war of communication," aviation analyst Gerard Feldzer said in Paris. "Everyone is pushing a pawn."
All 283 passengers and 15 crew members aboard the Amsterdam-to-Kuala Lumpur flight were killed in Thursday's crash.
CBS News national security correspondent David Martin reports that Pentagon officials say the missile likely came from a SA-11 from a position close to the border, but probably on the Ukrainian side of the border in territory held by the separatists. What is not known is the degree of Russian involvement.
Ukraine and the insurgents blame each other.
The U.S. ambassador to the United Nations, Samantha Power, said Washington believes the plane was likely downed by an SA-11 missile fired from an area controlled by pro-Russian separatists. She said Russia has provided the militants with SA-11s and other heavy weapons.
CBS News correspondent Margaret Brennan reported that surface-to-air missiles are at the very top of the list of weapons that the U.S has been trying to get Russia to stop providing to the pro-Russian separatists in Ukraine.
The U.N. Security Council called Friday for "a full, thorough and independent international investigation" into the downing of the plane, but that is a complicated proposition.
According to defense expert Anthony Cordesman of the Center for Strategic and International Studies, General Philip Breedlove, NATO Supreme Allied Commander Europe, warned just last month that Russia was training the Ukrainian rebels "to have an 'anti-aircraft capability.'"
"What we see in training on the east side of the border is big equipment, tanks, APCs [Armored Personnel Carriers], anti-aircraft capability, and now we see those capabilities being used on the west side of the border," Breedlove said, according to an article published by Cordesman on Friday.
Under international civil aviation rules, Ukraine should take the lead in investigating an airline accident on its territory. Anton Gerashenko, an adviser to Ukraine's interior minister, told The Associated Press that the investigation would be carried out by the Interior Ministry and the Security Services of Ukraine, who would work alongside international observers.
It was unclear what access either group would have to the crash site.
The U.S. National Transportation Safety Board (NTSB) said Friday that it was sending an investigator to Ukraine to assist in the probe, but the timing was still being worked out. Two U.S. law enforcement officials told CBS News, meanwhile, that a small team from the FBI was also heading for Ukraine to assist, but not as part of any criminal investigation.
The FAA is not yet sending a representative to help out with the crash investigation but "stands ready to assist as well, should the need arise," the agency said.
A 30-strong delegation, made up mostly of officials from the Organization for Security and Cooperation in Europe, traveled to the crash site Friday afternoon. But at the village of Hrabove, rebel militiamen only allowed the OSCE team to perform a partial and superficial inspection.
While the delegation was leaving under orders from armed overseers, two Ukrainian members lingered to glance at a fragment of the plane by the side of the road - only for a militiaman to fire a warning shot in the air with his Kalashnikov rifle.
OSCE spokesman Michael Bociurkiw, who was part of the team, said he was "shocked" to see that bodies were still lying in the open.
European Union officials said Friday that Ukraine has first claim on the plane's two black boxes - a flight data recorder and a cockpit voice recorder - which could contain valuable clues about what happened in the moments before the crash.
An assistant to the insurgency's military commander said Friday that rebels had recovered multiple devices from the wreckage and were considering what to do with them, raising fears they could be headed to Moscow. But Russian Foreign Minister Sergey Lavrov said Russia had no intention of getting hold of the boxes, and insurgent leader Aleksandr Borodai later contradicted his colleague and said the rebels don't have them anyway.
Defense experts said the plane was likely shot down by a missile fired from a Buk system, Soviet-era equipment that is in the arsenals of both Russia and Ukraine. There was no previous evidence of separatist rebels using such missiles, though a rebel Twitter account boasted last month about seizing a Buk system from Ukrainian forces, and AP journalists saw such a system hours before the crash Thursday in rebel-held territory.
Feldzer, the air-accident expert, said investigators' goal would be to "find the debris of the missile in question and determine the trajectory." Once investigators reach the site, they should be able to discover whether the plane was hit by one or more missiles, and the size of the missile system involved.
But, he said, "that won't determine who did it," unless investigators can find a satellite photo or radar records of the missile.
Justin Bronk, a research analyst at the Royal United Services Institute, a London-based military think tank, said the United States has sophisticated military satellites capable of detecting a missile launch, but might be unwilling to share its images in order to protect its secret surveillance capabilities.
"They will probably try to liaise with civilian satellite operators to see if there are any who also picked up the trail on infra-red sensors so that they can publicly release that data," he said.
The crash site is vast, and experts said rebel fighters may have removed key evidence in the chaotic hours after the disaster, as they joined emergency workers and local coal miners to comb the wreckage and recover the bodies.
Charles Heyman, editor of "Armed Forces of the EU," said missile casings could help establish who had supplied the weapons that brought down the plane. But he said it was likely that the rebels - if they fired the missile - would have removed any missile-casing debris from the scene.
Heyman said the missile launcher would bear ID numbers that could establish whether it was recently supplied by Russia or came from Ukrainian forces.
But he said if rebels mistakenly targeted a commercial airliner, thinking it was a Ukrainian military plane, they may have subsequently fled and taken the missile launcher into Russia.
The Ukrainian Interior Ministry released video purporting to show exactly that: a truck carrying a Buk missile launcher with one of its four missiles apparently missing, rolling toward the Russian border. The ministry said the footage was captured by a police surveillance squad at dawn Friday. There was no way to independently verify that claim.
"If I was the rebel chief of staff, I'd have had it taken away, dismantled and blown up," Heyman said, "and then bury the pieces in a swamp."