Rescue officials say about 100 neighbors converged to help find three people buried Friday when an avalanche swept down a mountain in a residential area of Missoula in western Montana and crushed a house at the bottom.

“It was very chaotic but a lot of energy,” said Jeff Brandt, assistant chief of operations for the Missoula Fire Department.

Scores of neighbors had already started the rescue effort when he arrived about half an hour after the slide, and some 20 professional responders helped provide focus to the effort, Brandt said. An 8-year-old boy was pulled from the snow just as he arrived, he said.

The three people remained hospitalized Saturday, a day after the avalanche slid down 4,768-foot Mount Jumbo into the northeast Missoula neighborhood, a St. Patrick Hospital spokeswoman said.

Fred Allendorf, 66, a retired professor from the University of Montana, is in serious condition while his wife, Michel Colville, is in critical condition, hospital spokeswoman JoAnn Hoven said. The boy, who hasn’t been named, is in fair condition.

The avalanche was 1,800 feet long and about 300 feet wide at the crown, said Steve Karkanen, director of the Missoula-based West Central Montana Avalanche Center. It came down a 35-degree slope and was then funneled into a gully, where it picked up energy, he said.

Karkanen estimated its speed at 120 to 180 mph when it struck the house.

The couple was in the home while the boy was playing outside when the avalanche hit about 4 p.m., Brandt said.  “The avalanche had completely destroyed the house, swept it completely off its foundation and crushed it,” he said.

The boy’s mother and sister witnessed the avalanche, but they weren’t caught in it, Brandt said. The snow was 20 feet deep in places after the avalanche, Brandt said.

Rescuers determined the home was occupied when the avalanche occurred, and they formed a line and pushed probes into the snow.

Not long after Brandt arrived, Allendorf was found under about 6 feet of snow in an air pocket in the remnants of the building.

“He was panicked,” Brandt said. “He had been buried under snow for about 45 minutes.”

Allendorf told rescuers he was within a couple feet of his wife when the avalanche struck, Brandt said. Rescuers continued looking and found Colville 30 yards from where they found Allendorf.

But Brandt said she wasn’t in an airspace, wasn’t talking and wasn’t responsive. “She was completely buried in snow,” he said.

The Missoula Police Department is investigating and looking for witnesses as to what caused the avalanche. Police didn’t return a call from The Associated Press on Saturday.

But Karkanen, who regularly investigates the causes of slides, said a snowboarder triggered it. The snowboarder was briefly caught in the avalanche himself but got himself out of it before it picked up speed, he said.

“It was an unsurvivable avalanche,” Karkanen said. “He certainly would have been ground up had he been caught up in the avalanche.”

Mount Jumbo is a winter game range managed by the city of Missoula and is closed to people in the winter, Karkanen said. The snowboarder came down the surface of the avalanche and helped with the rescue, and he also talked with police about what happened, he said.

“He wasn’t trying to hide anything,” said Karkanen, adding the snowboarder was likely unaware of local rules involving Mount Jumbo.

The avalanche in that area was extremely unusual, and likely took a unique series of events that have probably happened in the distant past but not since the valley had been settled, he said.

A thawing event last week caused the surface snow to melt and then freeze, forming an ice surface, Karkanen said. That was followed by more snow and winds that blew snow from the east face of Mount Jumbo to the west face, creating the conditions for the avalanche.

“It was a wind-loaded pocket,” Karkanen said.