By Laura Zuckerman
March 1 (Reuters) - An avalanche that destroyed a house and injured three people in Missoula, Montana, may have been inadvertently triggered by a snowboarder riding a mountain slope that was closed for recreational use, federal authorities said on Saturday.
The avalanche on Friday roared down Mount Jumbo and through a neighborhood northeast of downtown Missoula, several blocks from the University of Montana.
A snowboarder riding a slope on Mount Jumbo triggered the avalanche, according to a report issued by the U.S. Forest Service avalanche center in Missoula.
But Missoula police Sergeant Travis Welsh said investigators are still examining whether human activity like snowboarding caused the slide and whether the activity violated any laws.
The massive snow slide flattened a two-story house, burying its occupants and a boy in a nearby yard. Emergency responders and volunteers used probe poles to locate the injured beneath the snow and dug them out with shovels.
A married couple trapped for hours in the rubble of their home, Fred Allendorf, 66, and Michel Colville, 68, were in serious condition and critical condition respectively at St. Patrick Hospital in Missoula, a spokeswoman for the facility said on Saturday. The 8-year-old boy, whose name was withheld, was listed in fair condition.
The snowboarder was not injured in the incident and had not been arrested, Welsh said.
Large, destructive slides known as slab avalanches or "white death" are rare in populated areas of the U.S. West. Among avalanches that affect people, nearly all occur in the snow-covered high country and are mostly triggered by snowmobilers, skiers and snowboarders who are themselves injured or killed by the slide, according to federal avalanche experts.
The slope the snowboarder was riding is closed each year from Dec. 1 to March 15 to provide winter range for foraging elk, according to Missoula officials. Safety concerns tied to additional snowfall and high winds on Saturday led the mayor to close all Mount Jumbo slopes and much of a mountain overlooking the university and to ask residents to report violators.
The massive slide in Missoula comes during a season that has seen avalanches claim 17 winter recreationists in the U.S. West - nearly twice as many recorded during the same period last season - drawn to backcountry slopes in search of abundant fresh powder.
Half of all avalanche victims completely buried in snow die within the first 25 minutes and 95 percent are dead within two hours, U.S. avalanche experts say. The vast majority of victims die from asphyxiation, the result of being cut off from a fresh supply of air under the snow. (Reporting by Laura Zuckerman in Salmon, Idaho; editing by Kevin Murphy and Diane Craft)