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COPPER, Colo. — An avalanche forced the closure of State Highway 91 between Copper and Leadville Thursday afternoon.

The Colorado Department of Transportation said four vehicles were buried in the slide. All of the vehicles’ occupants are safe.

Highway 91 will remain closed overnight, according to CDOT. The agency said the slide was natural and occurred on Resolute Cliff, a path that has never run before. CDOT said it had not done avalanche mitigation on the path in the past.