History of Space Exploration: Chapter 4, Exercise 3 — Possible Solution
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EXPLANATIONS GIVEN
Weather conditions, a possible bird strike, oxygen deprivation, or a near-miss
collision.

EVIDENCE NEEDED
Weather records for that day, the wreckage and flight data, the aircraft's
oxygen system, and radar or witness accounts of other aircraft. The chapter
says the Kremlin declined further investigation in 2007.