By Jon Ostrower and Andy Pasztor 

International air-accident investigators have yet to reach the farmland crash sites in eastern Ukraine to analyze the scattered remains of a Boeing Co. 777 or the jet's black boxes, more than three days after Malaysia Airlines Flight 17 was shot down by what U.S. and Ukrainian officials have said was a surface-to-air missile.

While evidence mounts that the plane was shot down by a sophisticated antiaircraft weapon, both forensic evidence at the scene and the data- and cockpit-recording devices, known as black boxes, remain crucial to understanding how the jet came apart and crashed. Debris could point to clues about where any missile hit.

The black boxes could also definitively rule out any equipment malfunction on the jet itself. They could also provide some indication of the unlikely chance that the pilots saw a launch or knew a missile was approaching.

The jet's twin data recorders are now embroiled in the region's volatile politics. Ukrainian government officials said on Sunday that they had intercepted conversations suggesting separatists were under pressure from Moscow to find the black boxes and keep them out of the hands of international investigators.

Video taken at the scene appears to shows one uniformed recovery worker carrying a day-glo-orange box, purported to be one of the flight-data or cockpit-voice recorders.

On Sunday, the separatist leader of the self-proclaimed Donetsk People's Republic, Alexander Borodai, said the black boxes had been located and were being stored in Donetsk. He said he was ready to hand them over to international aviation experts.

"We are not tech experts; none of us have seen a black box. We suspect that the artifacts we've recovered are the black boxes," he said.

Officials from the air-safety arm of the United Nations are proposing to take possession of the black boxes, said people familiar with the details.

A team of investigators from the International Civil Aviation Organization arrived in Kiev on Sunday night, these people said, and immediately began discussions to be designated as the impartial organization that will safeguard the flight-data and cockpit-voice recorders, or black boxes. No final agreement has been reached, they said, as ICAO works with other international groups and Ukrainian officials to develop a strategy.

Early Monday in Moscow, Russian President Vladimir Putin put his weight behind such a deal.

"It's essential for a robust team of experts to work on the site of the crash under the auspices of ICAO, the relevant international commission, " he said on the Kremlin website posted early Monday.

Another person familiar with the details said Kiev reached out to ICAO on Thursday for help, and it agreed to assemble a team to tackle the issue of gaining control of the black boxes. The team includes representatives of the U.S. Federal Aviation Administration, the U.S. National Transportation Safety Board and the U.K.'s Aircraft Accidents Investigation Branch, this person said.

On top of Ukraine's request for help, the U.N. Security Council, which meets on Tuesday, could formally call for ICAO to take a direct leadership role in the probe. The ICAO delegation, said several people close to the developments, is led by Marcus Costa, the agency's chief accident investigator.

After securing the black boxes, the team of experts working under ICAO's purview are expected to coordinate efforts to secure the wreckage. In addition, said one person familiar with the plan, the team's broader responsibilities are slated to extend to overseeing retrieval of information from the black boxes; analyzing radar data and satellite imagery; and setting up further teams of structural experts and other investigators to look for and analyze missile fragments.

Once air-crash investigators are provided unfettered access to the crash site, even a preliminary examination of debris can yield important clues about what likely brought down a plane. The type of damage detected on the surface of the metal skin, and where pieces of the wreckage ended up in relation to each other, are the major elements of such an analysis.

"It's relatively easy to see evidence of the punch inward from a missile strike versus the outward burst caused by some explosion inside the plane," said Bill Waldock, a structures expert who teaches at Embry-Riddle Aeronautical University.

Images of debris from the scene, Mr. Waldock said, indicate that the tail section of the aircraft, including the horizontal stabilizer, most likely separated from the fuselage while the Boeing 777 was still airborne. Much of the rest of the plane--including the wings, main landing gear, significant parts of one or both engines and the center section of the fuselage where the wings are attached--appears to have ended up in close proximity.

"Whatever happened blew the tail off first, while the rest of the aircraft remained more or less intact" until it hit the ground, Mr. Waldock said.

The debris field also can provide clues about the type of missile and even the direction from which it came, according to air-safety investigators. An SA-11 missile, such as the one U.S. and Ukrainian officials believe was fired at the Boeing 777, can operate off a proximity fuse that would make the warhead explode before striking its target.

The impact from shrapnel is designed to sever an aircraft's fuselage, immediately destroying essential systems and making it impossible to fly.

When international investigators examined a U.S. missile strike that brought down an Iranian passenger jet over the Persian Gulf in 1988, they were able to document precisely such a sequence of events.

By examining the wreckage and other data, investigators determined the angle of the missile strike, said Caj Frostell, former chief accident investigator for the International Civil Aviation Organization. Mr. Frostell worked on the 1988 investigation.

Despite the widespread presumption that a missile strike brought down Flight 17, careful analysis of the debris is still necessary for investigators to conclusively document how the plane came apart.

"Some people would say at this point, that there is not much doubt about what happened," Mr. Frostell said. But debris analysis is necessary as part of the definitive answer, he said.

However, clues into what exactly brought down MH17 could be increasingly difficult to identify. Malaysia's transport minister, Liow Tiong Lai, said the country is "very concerned the sanctity of the crash site has been severely compromised."

Data downloaded from the plane's black boxes, if they end up in the possession of investigators, could buttress explanations of where the plane was hit and what sections fractured first.

Such data would show a cascading series of equipment failures aboard the plane, air-safety experts said, even as it was likely spiraling out of control and plummeting toward the ground, over a period spanning minutes, from 33,000 feet where the jet had been cruising.

Alexander Kolyandr and Richard C. Paddock contributed to this article.

Write to Jon Ostrower at jon.ostrower@wsj.com and Andy Pasztor at andy.pasztor@wsj.com

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