MOSCOW--When investigators eventually sort through the wreckage to determine why Malaysia Airlines Flight 17 was shot down, they will have to consider the plane's tragic trajectory: straight over the center of some of the hottest fighting in eastern Ukraine, where air power had begun playing a crucial role on the battlefield.

The plane entered that dangerous zone just hours after rebels had gotten their hands on an antiaircraft missile system sophisticated enough to take down a plane cruising at 33,000 feet, U.S. and Ukrainian officials say. That missile system, the Buk, known in the West as the SA-11 or Gadfly, is particularly inept on its own at distinguishing a civilian aircraft from a military one, according to military experts.

Russia says Ukraine, not rebels, downed the Malaysian flight with a Buk missile from its own arsenal, or in an air-to-air engagement with a Ukrainian fighter jet. The U.S., citing Ukrainian evidence that the missile battery and its crew were brought over the border from Russia, say the incident illustrates the widening danger of Moscow's willful proliferation of weapons in the Ukrainian battle zone.

The path of the Malaysia Airlines flight took it directly over a stretch of the Russia-Ukraine border near the town of Snizhne, where fighting had raged for weeks. The territory was critical for both sides, because Ukrainian troops had effectively cut off rebels from Russia by seizing and holding a thin strip of territory along that border.

Air power has become a factor tipping the war in Ukraine's favor in recent weeks, not only as a means to strike rebel positions, but also to spot targets for artillery and missiles. Artillery barrages have been a vital tool for the Ukrainian military in making gains against separatists forces, a U.S. official said: "Not the only tool, but an important one." In general, aerial observation can dramatically improve the accuracy of effectiveness of long-range artillery, he said.

The rebels have no air force, although Ukrainian officials say they have been operating some drones. Intercepted radio transmissions released by the Ukrainian government have shown rebels prior to the downing of the Malaysian flight were having trouble using their own artillery effectively because they didn't have forward observers for their guns. Lately, rebels have tried to even the score by acquiring antiaircraft weapons that can reach to higher altitudes.

In early fighting, rebels were armed with primitive point-and-shoot antiaircraft guns. But in May Ukrainian officials noted the appearance of shoulder-launched rocket systems, which the rebels used to take down helicopters and low-flying planes.

But earlier this month a video of a rebel column of military hardware in the city of Donetsk included several SA-13 Gophers, an infra-red tactical-level guided missile system that could take down planes as high as 13,000 feet. Ukrainian officials began reporting the loss of some higher-flying aircraft, such as fighter jets and Antonov cargo planes after the appearance of SA-13s.

Ukraine continues to lose lower-flying ground attack aircraft to missiles launched by the rebels. On Wednesday, the Ukraine military reported that two ground attack Su-25s were shot down at 17,000 feet not far from the Flight 17 crash site.

For reconnaissance flights, analysts say the Ukrainians probably tried sending their planes higher and out of reach of the rebel antiaircraft systems. The delivery of the Buk system appears to have been a Kremlin gambit to give rebels a chance to shoot down the highest-possible planes that the Ukrainians could fly over them. A question now is why its crew unleashed a missile against a civilian aircraft.

Military experts say the Buk is ideally deployed as just one component of a wider antiaircraft system, since it doesn't have the capabilities to search the skies over a wide area. Doug Richardson, editor of IHS Jane's Missiles & Rockets, said that while the Buk operating alone can identify a friendly aircraft, it can't distinguish between enemy or neutral ones.

Recordings of allegedly intercepted telephone conversations released in recent days by the Ukrainian security services suggest that rebel crews operating the Buks were concerned about artillery strikes--and relieved that the Buks had been delivered to them.

About seven hours before the Malaysia flight fell from the sky, Ukraine's security service said, it intercepted a phone call from a pro-Russia militant to a rebel commander. According to the recording, the rebel called the Buk "a beauty" that he had just brought from across the Russian border on a flatbed truck. The missile system, he said, came with a crew to operate it, according to the recording.

Ukraine's security service identified the rebel commander as a former Russian military officer, Sergei Petrovsky. The caller told Mr. Petrovsky that "we need to unload it somewhere and hide it," according to the recording.

Mr. Petrovsky told him to rendezvous with an armored column that would escort the system outside the city. Within two hours, it had been deployed, Ukraine security officials said.

Shortly after issuing orders to deploy the system, Mr. Petrovsky complained to another rebel commander about the "constant" Ukrainian artillery fire, saying "only now there is a pause."

When asked why he didn't fire back with their own Grad system, Mr. Petrovsky replied: "The thing is that we have a Grad, but don't have a spotter, firstly," according to the released recording.

"Thank God, today Buk M arrived," he is heard saying on the recording. "It has become a little bit easier."

Write to Alan Cullison at alan.cullison@wsj.com

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