Officials said on Thursday that it could weeks to determine what caused two helicopters to collide in midair Wednesday night during routine military exercises along the Arizona-California border. Six of the dead were based at Camp Pendleton in California, and one was from a base in Yuma, Ariz. The skies were clear, but a spokesperson from the Yuma base said visibility may have been severely limited by dust, referred to as a "brown out," created by the helicopters landing and taking off. The crash is one of the Marine Corps' deadliest aviation-training accidents in years.
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