Nevada County Sheriff Shannan Moon announced the deaths at a news conference Wednesday.
Search and rescue crews were dispatched to the Castle Peak area of the Sierra Nevada after a 911 call Tuesday afternoon reporting an avalanche had buried 15 skiers. Six of them have been found alive.
It is the deadliest avalanche in the U.S. since 1981, when 11 climbers were killed on Mount Rainier, Washington.
The group was on a three-day trek in Northern California's Sierra Nevada on Tuesday morning when the avalanche occurred as a monster winter storm pummeled the West Coast.
Two of those rescued after several hours of searching were taken to a hospital for treatment, said Ashley Quadros, a spokesperson for the Nevada County Sheriff's Office. Heavy snow and the threat of additional avalanches slowed the rescue effort in the mountains near Castle Peak, northwest of Lake Tahoe.
The area near Donner Summit is one of the snowiest places in the Western Hemisphere and until just a few years ago was closed to the public. It sees an average of nearly 35 feet of snow a year, according to the Truckee Donner Land Trust, which owns a cluster of huts where the group was staying near Frog Lake.
The Sierra Avalanche Center warned Wednesday that the risk of avalanche remains high and advised against travel in the area. Multiple feet of snowfall and gale force winds in recent days left the snowpack unstable and unpredictable, and more snow was predicted to fall, the center said.
