Nine skiers are still missing after an avalanche swept the Castle Peak area of the Sierra, Nevada, mountains in California. Authorities said six others, who had been stranded, have since been rescued.

The avalanche occurred about 10 miles north of Lake Tahoe at about 11.30am on Tuesday, engulfing a group of backcountry skiers – including four guides and 11 clients.

The Nevada county sheriff’s office said in a Facebook post late Tuesday that the six who survived and had been rescued had taken refuge in a makeshift shelter, constructed partly from tarpaulin sheets. The office communicated with rescuers via radio beacon and text messaging.

“Due to extreme weather conditions, it took several hours for rescue personnel to safely reach the skiers and transport them to safety where they were medically evaluated by Truckee Fire,” it said. “Two of the six skiers have been transported to a hospital for treatment”.

“The search is ongoing, pending weather conditions,” the statement added. The sheriff’s office revised the number of people in the group to 15 from an earlier estimate of 16.

Reports have noted that if all nine missing skiers should perish, the avalanche would rank among the deadliest single avalanches on record in the US. Conditions in the area this week, with heavy snowfall, strong winds, and low visibility have created what scientists at the Central Sierra Snow Lab called some of the worst conditions the region has experienced in years.

The Sierra Sun said that ski rescue teams had been dispatched from the Boreal Mountain ski resort and Tahoe Donner’s Alder Creek adventure center to make their way to the known survivors. The sheriff’s office said 46 emergency first responders had joined the search party.

The Colorado avalanche information center has tallied six US avalanche deaths so far this season. It says avalanches have claimed an average of 27 lives over the past decade in the US.

Tuesday’s avalanche near Truckee, California, occurred during a winter storm warning for much of northern California, with heavy snow forecast for higher elevations of the Sierra Nevada. The town of Soda Springs, near where the avalanche took place, recorded at least 30 inches of snow during a 24-hour period, according to the Soda Springs Mountain Resort.

The dangerous conditions were caused by rapidly accumulating snow piling on fragile snowpack layers coupled with gale-force winds.

The Sierra avalanche center had posted an alert before dawn on Tuesday, warning of a “high avalanche danger” in the ski region, the sheriff’s office said. The center lists five other avalanches incidents so far this season with six killed.

“It’s particularly dangerous in the backcountry right now just because we’re at the height of the storm,” Brandon Schwartz, Tahoe National Forest lead avalanche forecaster at the Sierra avalanche center, told the Associated Press.

Steve Reynaud, a Tahoe National Forest avalanche forecaster with the Sierra avalanche center, said the skiers were on the last day of a three-day backcountry skiing trek, which had contact with people on the ground in the area.

Reynaud told the AP that the skiers spent two nights at huts on a trip that required navigating “rugged mountainous terrain” for up to 4 miles while bringing along all food and supplies.

Russell Greene, the Nevada county sheriff captain, said authorities had been notified about the avalanche by the ski tour company that led the expedition.

“I don’t think it was a wise choice,” Greene told the AP of the decision of a ski company to take paying customers out into the backcountry under such conditions, adding, “but we don’t know all the details yet”.

Reuters reported that the area around Castle Peak, a 9,110-foot mountain north of Donner Summit, is a popular backcountry skiing destination. The summit is named for the infamous Donner party, a group of pioneers who resorted to cannibalism after getting trapped there in the winter of 1846-1847.

Gavin Newsom, the California governor, was briefed on the avalanche, and state authorities were “coordinating an all-hands search-and-rescue effort” in conjunction with local emergency teams, the governor’s office said on X.