NEW DELHI — The Sherpas always go first, edging up the deadly flank of Everest while international clients wait for days in the base camp below.

They set off in the dark, before the day’s warmth causes the ice to shift. They creep one by one across ladders propped over crevasses, burdened with food and supplies, all the while watching the great wall of a hanging glacier, hoping that this season will not be the year it falls.

On Friday, however, it did.

Around 6:30 a.m., as the Sherpas were tethered to ropes, a chunk of ice broke off, sending an avalanche of ice and snow down into the ice fields on the mountain’s south side and engulfing about 30 men. The toll, at 12 dead, with four still missing, is the worst in a single day in the history of Everest, climbers and mountaineering experts said.

The disaster has focused attention on the Sherpas, members of an ethnic group known for their skill at high-altitude climbing, who put themselves at great risk for the foreign teams that pay them. Among their most dangerous tasks is fixing ropes, carrying supplies and establishing camps for the clients waiting below, exposing themselves to the mountains first.