CHAMONIX, France — A climber trying to scale Mont Blanc may accidentally have caused a slab of ice to snap off Thursday high in the French Alps, sparking an avalanche that swept nine European climbers to their deaths, authorities said. A dozen climbers were injured and two were still missing by nightfall.

As a sheet of snow and ice thundered down the steep slope, several other climbers managed to turn away from the slide in time, regional authorities in Haute-Savoie said.

Two climbers were rescued as emergency crews using dogs and helicopters scoured the churned-up, high-altitude area in a frantic search for the missing. Their quest, hampered by the possibility of further avalanches, was called off by nighttime.

Three Britons, three Germans, two Spaniards and one Swiss climber were known to have died, the prefecture of the Haute-Savoie region said.

The dead included the former general secretary of the British Mountaineering Council, Roger Payne, the council said on its website. Current BMC head Dave Turnbull praised Payne as one of Britain's most notable climbers, with expeditions from the Alps to the Himalayas.

An initial report of four missing was lowered to two, and officials noted the numbers of those involved in the drama could vary because some climbers may have struck out on their own. A group of 28 was known to have left a mountain refuge for the ascent.

Close to 90 people were involved in the search.

Early summer storms apparently left behind heavy snow that combined with high winds to form dangerous overhanging conditions on some of the popular climbing routes around Mont Blanc, the highest mountain in western Europe. Regional authorities had warned climbers earlier this summer to be careful because of an unusually snowy spring.

The Mont Blanc massif is a popular area for climbers, hikers and tourists but a dangerous one, with dozens dying on it each year. Chamonix, a top center for climbing, hosted the first Winter Olympics in 1924.

Police said they were alerted around 5:25 a.m. Thursday to the avalanche, which hit a group of climbers — people from Switzerland, Germany, Spain, France, Denmark and Serbia — who were 13,100 feet high on the north face of Mont Maudit, part of the Mont Blanc range.