The backcountry ski trip hit by avalanche was led by an outfit offering mountain adventures globally

The group of 15 backcountry skiers headed out on a three-day trek organized by Blackbird Mountain Guides into pristine wilderness near Frog Lake in California's Sierra Nevada as a powerful winter storm moved into the state.

The skiers made their way Sunday to remote huts situated at 7,600 feet (3,415 meters) in Tahoe National Forest, carrying their own food and supplies. At 6:49 a.m. that same morning, the Sierra Avalanche Center had issued an avalanche watch for the area, indicating large slides were likely in the next 24 to 48 hours.

Around 5 a.m. Tuesday, the morning the group was scheduled to make its way back out to the trailhead, the center increased the watch to a warning. Within hours an avalanche hit near Castle Peak, trapping the skiers. Authorities said Wednesday that eight people were killed and one person remained missing. Six skiers were rescued after sheltering in place for hours as searchers battled blizzard conditions.

Nevada County Sheriff Shannan Moon said investigators were examining the decision to proceed with the trip Sunday despite forecasts of a major storm. It's not known if the guides would have known about the avalanche warning as they returned to the trailhead.

''We're still in conversation with them on the decision factors that they made,'' Moon said at a Wednesday news conference announcing the deaths.

Before the disaster, the tour company said its guides were highly skilled in dealing with extreme conditions and that it offered avalanche education.

''If you've booked the Frog Lake Huts in Truckee, trust our guides to elevate your trip to the next level,'' the business said on its website, advertising the trip. ''We'll navigate in and out of the huts, manage the risks, and find the best terrain and snow quality for you and your group!''

The company, with offices in California and Washington state, offers mountaineering and backcountry ski trips across the U.S. West Coast as well as in Europe and Japan. It also offers safety courses like wilderness skills and first aid.

The rugged terrain had been closed to the public for a century before the Truckee Donner Land Trust bought it and opened the huts, which offer basic accommodations such as sleeping pads, gas stoves and flush toilets.

The tour, which cost $1,165.00 per person, was rated for intermediate-to-expert skiers with at least 20 days of backcountry experience who can climb up to 2,500 vertical feet (760 meters) throughout a day, according to the company's website. Guides carry first aid kits, while participants must bring their own ski equipment and avalanche gear, including a beacon, shovel and probe.

''Generally our guides are able to find excellent backcountry snow conditions, but often we need to travel through difficult conditions,'' the website description said, adding participants must "be adept with their backcountry touring skills and have a solid foundation of touring before the trip.''

Experienced backcountry skiers can still navigate areas where much snow has fallen as long as they are familiar with the topography and terrain, said Brandon Mathis, a Durango, Colorado backcountry skier trained in avalanche safety. In those places, experts should know to avoid sending large groups too close together.

''Things get serious, needless to say, out there. So knowledge is one of your best tools for safety,'' Mathis said.

The Sierra Avalanche Center stated that rapidly accumulating snow piling on fragile snowpack layers, along with strong winds, contributed to the treacherous conditions this week. The town of Soda Springs, near where the avalanche took place, recorded at least 30 inches (76 centimeters) of snow in a 24-hour period, according to the Soda Springs Mountain Resort.

Anthony Pavlantos, owner of Utah-based Prival USA, makes avalanche safety equipment and runs mountain safety programs. He said heavy snowfall in 24 hours is a ''really big red flag'' and the safest option is often just to leaver rather than hunker down.

''High intensity snowfall in short amounts of time can increase the avalanche hazard fast," he said. "So, you already have a snowpack sitting on the snow and then you have all this new snow that's falling down at a high rapid rate. That new snow can slide on that old snow surface.''

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