TRUCKEE, Cali. — A new report is giving insight into the deadly avalanche that claimed the lives of nine people in the Sierra Nevada last week.

Authorities initially said that eight skiers were killed during the historic avalanche on Castle Peak, but the Sierra Avalanche Center confirmed that a ninth person reported missing was found dead a few days later.

The ninth skier was found buried in the snow prior to nightfall on Friday, Feb. 20, the avalanche center said.

A group of 11 clients and four professional guides from Blackbird Mountain Guides were on the final leg of their three-day backcountry excursion when the avalanche struck around 11:30 a.m. local time.

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In their initial report, the avalanche center said a group of 15 backcountry travelers were involved in the avalanche, which happened below Perry’s Peak on Tuesday, Feb. 17.

Twelve of those people were caught in the avalanche, and the remaining members of the party performed a companion search and were able to rescue three people in their group.

The survivors were able to shelter and contact emergency responders through the SOS function on an iPhone.

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When search and rescue crews arrived in the late afternoon, they excavated eight of the nine skiers who were buried under snow and confirmed them dead.

Crews worked in the darkness amid high-intensity storm conditions, and they safely recovered six survivors that night.

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Three days later, when the storm ended, avalanche mitigation was performed on the slope using PG&E helicopters with roughly 5,500-lb, 660 gallon water buckets that were placed and dragged on the slope. The helicopter made full-load water drops on the slope in many areas, according to the avalanche center.

After avalanche mitigation, search and rescue teams were able to remove five of the skiers found dead, and that’s also when the ninth skier was found.

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Rescue operations concluded on Saturday, Feb. 21, once the final four skiers’ bodies were recovered.

The avalanche in the rugged Donner Pass region of California‘s northern Sierra marks the deadliest U.S. avalanche since the 1981 disaster on Mount Rainier that claimed 11 lives.

Shortly after the avalanche, Tahoe National Forest announced that all national forest lands and trails in the Castle Peak area are closed. They expect the closure to remain in effect through mid-March.

According to the FOX Forecast Center, 30 mph gusts were recorded at the time of the avalanche.

In a statement, the ski company said all the guides were either part of the American Mountain Guides Association or certified in backcountry skiing.