Black box data from Malaysia Airlines Flight 17 indicates that the plane crashed as a result of a "missile explosion," Ukraine defense officials said on Monday.

Pro-Russian rebels, who were early on scene after the crash and closely guarded the site, turned over the black boxes to Malaysian authorities last week in the eastern Ukrainian city of Donetsk. The flight data and cockpit voice recorders were then sent to the UK for analyzation.

Andriy Lysenko, spokesman for the Ukrainian National Security and Defense Council, said the experts who studied the data told him that the plane crash was caused by "massive explosive decompression as a result of the fragmentation and missile explosion."

Though the cockpit voice recorder was damaged in the crash, the data was still intact, and there was no evidence that the separatists had tampered with the device, as Western officials had feared.

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Since the Malaysian plane was shot down on July 17, fighting in eastern Ukraine has escalated. The violence has now spilled over into the cities and residential areas. As a result, civilian casualties are on the rise, and many people have fled their homes.

The clashes have particularly increased near the crash site, which is about 40 miles from Donetsk, making it dangerous for international teams to investigate what little remains of the downed Boeing 777 that was carrying 298 people.

A team of international police officers on Sunday had planned on searching for the evidence and the remaining victims' bodies. However, they cancelled their trip after reports of fighting in the area, which Lysenko confirmed in his press conference on Monday as well.