NBC News — French investigators say a deadly mountaineering accident on a slope leading to the famous Mont Blanc was probably caused by wind. Early thursday morning nine climbers were killed when they were swept away by an avalanche. Officials say the climbers were hit by a wall of snow some 20 feet tall.

A preliminary investigation suggested a climber may have stepped on an icy patch, unwittingly triggering the giant slide. Many of the climbers were roped together for safety reasons. Eleven others were injured and have been hospitalized, four other climbers are still missing.