Three climbers were killed Sunday in an avalanche on Colchuck Peak in Washington, authorities said. The tragedy—which is the deadliest avalanche to have occurred in the U.S. this winter—occurred as a group of six climbed a couloir on the mountainside. Chelan County Sheriff’s Sgt. Jason Reinfeld said the lead climber triggered the snow slide, which swept up four of the group and carried them around 500 feet down the rocky gully. Law enforcement believe two were killed by the trauma of the avalanche and a third died shortly after. The fourth person survived with knee and ankle injuries. A seventh member of the group who stayed at camp instead of participating in the climb hiked through snow for around 5 miles to get help early Monday.