A snowboarder triggered the avalanche that buried three people — including an 8-year-old boy — in a home in a residential neighborhood of Missoula, Mont., investigators said Saturday.
St. Patrick Hospital identified two of the victims: Fred Allendorf, 66, a retired biology professor, and his wife, Michel Jo Colville, 68, an artist. The hospital told NBC News that Colville remained in critical condition, while Allendorf had been upgraded to serious condition.
The 8-year-old boy, identified as Phoenix Scoles, was listed in fair condition and was expected to recover.
Police, fire officials and search-and-rescue crews from around the region worked for more than three hours to recover the three after the wave of snow washed over two homes on Holly Street in Missoula's Rattlesnake Valley neighborhood about 4 p.m. (6 p.m. ET) Friday. Rescuers said Saturday that the victims were able to survive only because of air pockets in the debris.
The West Central Montana Avalanche Center said the incident was caused when a snowboarder in a restricted area near the top of Mount Jumbo disturbed a layer of packed snow resting on looser snow beneath it. The snow rapidly tumbled down the mountain, gathering speed and bulk in what the center called a slab avalanche — the most dangerous kind.
"That continuous snowfall kind of keeps the snowpack at its tipping point, or breaking point, where it just takes the added weight of a skier or snowmobiler to tip the balance, and you get an avalanche," said Mark Staples, an avalanche specialist with the Gallatin National Forest Avalanche Center.
Authorities were investigating, and the snowboarder hadn't been identified or charged Saturday evening.
The Missoula Valley, Mount Jumbo and Mount Sentinel remained under a high avalanche danger advisory Saturday, indicating that natural avalanches were possible and that further human-triggered avalanches were likely.
No mandatory evacuations had been ordered, but many families had vacated on their own. Missoula police told NBC station KECI that they were going door to door to warn other residents of the danger.
Ron Allen, Brittany Auger and Sossy Dombourian of NBC News contributed to this report.