Nine Europeans including three British climbers were killed in an avalanche that could have been sparked by a single falling ice slab, while they attempted to climb Mont Blanc in the Alps.
The disaster, which followed a period of unseasonably hot weather, resulted in 11 other climbers being taken to hospital. Four more are still unaccounted for.
Along with the Britons, two Germans, two Spanish and two Swiss climbers were killed. They were in a group with climbers from France and Serbia when they were stuck by the wall of snow.
Police from the French ski resort town of Chamonix believe the avalanche could have been sparked by a climber knocking free a slab of ice and causing a landslide that smashed into the mountaineers 4,000 metres (13,000ft) up the north face of Mont Maudit - the so-called "cursed mountain" - in the Mont Blanc range.
Swept off the mountain face
The gendarmerie told Associated Press that the avalanche appeared to have been caused by a 40cm-thick block of ice breaking off and sliding down the mountainside.
It set off a chain reaction and grew into a mass of snow and rock that was 2 metres thick and 50 metres long. The climbers, who will have been tied together in groups, were swept off the mountain face.
Col Bertrand Francois of the Haute-Savoie gendarme service said: "The first elements that we have from testimony are that a climber could have set loose a sheet of ice, and that sheet then pulled down the group of climbers below. I should say that the incline was very, very steep on this northern face.
The climbers are thought to have set off from Refuge des Cosmiques and the accident happened two-and-a-half hours later.
Dozens of police and other rescue workers used dogs to find survivors and the dead in high altitudes and through tonnes of snow.
Jonas Mowstrup, a Danish climber, told Ritzau news agency that he heard about the accident when he was making his way down Mont Blanc.
Overhanging slabs of ice
He said: "Three days ago we ascended [Mont Maudit]. it was shocking to hear, it could have easily been us. It is scary and tragic, [but] it's part of the thrill that something can go wrong."
Warnings had been issued to climbers about the unusually snowy period, while in the week leading up to the tragedy, climbers had tweeted that high winds had created overhanging slabs of ice forming on the northern slope. The climbers who were hit are understood to have been an experienced group, although some were with professional guides.
French investigators have been called in. French interior minister Manuel Valls paid tribute to the rescuers.
He said: "I admire their drive in the difficult conditions in which they have been working - they're quite perilous.
The Mont Blanc massif is a popular climbing spot, with hundreds taking on its challenging peaks every year. Mont Maudit, which in French means "Cursed Mountain", is the third highest peak in the range.