With a thunderous roar that echoed through the Canadian Rockies, a 100-foot-wide tide of snow swept him off his skis and propelled him down the mountain. He fought, swimming in the snow, not knowing which way was up. Seconds later, his body came to rest, trapped up to the neck.

"It's like being in white water until it stops, and then it's like being in concrete," said John Seibert, a survivor of an avalanche that killed four Canadians and three Americans on Monday as they skied the backcountry of British Columbia.

"I've been through avalanche training courses," he told reporters. "You think you know what you're going to do. . . . I don't know if what I did was right or not, but I think everyone on that mountain that day was extremely heroic."

Seibert, a geophysicist from Alaska, told reporters in British Columbia that it took him 20 minutes to dig himself out, but that he was not injured. Once he freed himself, he began searching for other skiers. "In all, 13 of us were caught in the slide. Seven of our friends were killed."

The police are still investigating the cause of the accident, which involved 21 skiers. Police said experts had been at the scene to try to determine the condition of the snow pack at the time of the avalanche.

"There is nothing in the initial investigation . . . to lead investigators to believe that this is [anything] other than a tragic accident," the police said in a statement. The coroner will probably require several months to finish the report, officials said, and will then decide whether to hold an inquest into the accident.

Police said the avalanche occurred about 10 a.m. in the area known as the Durrand Glacier, about 34 miles northeast of Revelstoke, B.C.

A group of 24 had left for a day of backcountry skiing organized by Selkirk Mountain Experience Ltd., but three members of the group decided to stay behind at a remote mountain lodge.

The leader of the remaining 21 who went out was a well-known guide, Ruedi Beglinger, whose advertisements say he operates demanding tours that require skiers to be fit enough to climb nearly vertical mountains. Some of the skiers carried beacons for use in an emergency.

Police said the group split into two, one going higher than the other, and were skiing across a 35-degree slope when the avalanche, with a path that was about 100 feet wide and 300 feet long, began. The lower of the two groups was hit by the snow, which buried eight skiers and nearly covered three others. The three were able to break out of the snow and free one of the other eight.

Beglinger and the three skiers who were not buried in snow made a futile search for their seven missing compatriots. Their bodies were found later under about three to four yards of snow, police said. They were taken back to Revelstoke on Monday afternoon and placed in a makeshift morgue.

Police identified the victims as Kathleen Kessler, 39, of Truckee, Calif.; Ralph Lunsford, 49, of Littleton, Colo.; Dennis Yates, 50, of Los Angeles; Naomi Heffler, 25, of Calgary; Dave Finnerty, 30, of New Westminster, B.C.; Jean Luc Schwendener, 40, of Canmore, Alberta, and Craig Kelly, 36, of Nelson, B.C.

Kelly was known worldwide for pioneering snowboarding. He had won four world championships in the sport.

A paramedic who helped recover the victims told reporters that when he arrived at the scene, the avalanche pack was solid. "You could walk on it, just like this table," said Ian Stratham, of the ambulance service in Revelstoke. "It doesn't take long to asphyxiate. If you get buried in one of those, you can barely move a muscle."

Clair Israelson, managing director of the Canadian Avalanche Center in Revelstoke, said that when the skiers headed out, avalanche conditions were rated "considerable" but were not unusual.

Avalanches are usually caused when a buried layer of snow melts and weakens the pack. A loud noise or a tremor can trigger an avalanche, with the snow traveling up to 60 mph. An avalanche can uproot trees, crush buildings and flip cars.

Avalanches kill an average of 10 people each year in British Columbia. In 1998, Michel Trudeau, the 23-year-old son of former prime minister Pierre Trudeau, died when an avalanche pushed him into an icy lake during a backcountry ski trip in Kokanee Glacier Park, near Nelson, B.C. Michel's older brother, Justin, began an awareness campaign to warn people of the dangers of avalanches.

Pierre Trudeau died in 2000.

Seibert, a survivor of Monday's avalanche, said it came with no warning. "I feel that this tragic accident was a fluke of nature," he said. "There was nothing in my mind that was a warning sign that we should not be skiing on that slope on that day. The tragic loss of our friends will haunt us for the rest of our lives."