The seven high school students killed in an avalanche in southeastern B.C. on Saturday all died of asphyxiation, according to the B.C. coroner.

At a briefing details of the power of the 500-metre-wide slide emerged.

It was an avalanche of incredible power. In just four seconds, it slid one kilometre and was strong enough to destroy homes or wreck a train.

(CP PHOTO)

Eric Dafoe, the public safety co-ordinator at Glacier National Park, says two local experienced skiers saw the avalanche coming and shouted down to warn the young group.

One "was able to yell out 'Avalanche, avalanche, avalanche,' three times to warn them. But he felt there was just a matter of seconds before the dust cloud engulfed them," he said.

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Students arrive outside Strathcona-Tweedsmuir School in Okotoks, Alberta Sunday (CP PHOTO)

All 17 in the group were partially or fully buried. Coroner Dave Purse says the six boys and one girl all died of asphyxiation.

For Purse, it's the second time in two weeks he's had to face such a tragedy. "It's very tough. It's very difficult. It's 14 deaths in two weeks here."

It raises the question why young students were even on the slope. The risk of avalanche was rated "considerable." That means natural avalanches are possible.

Meanwhile, a small school community south of Calgary is grieving after the tragic deaths of the students. The students were taking part in an annual outdoor education trip in a well-skied area of the Rogers Pass.

"Although we still do not have all the details we are absolutely stricken with grief. Our school is a small family and a loss this extreme is impacting on us tremendously. No doubt this will have ripples throughout Southern Alberta, and our hearts go out to all of those who have been impacted in this way," said school principal Glenn Odland.

The seven students were all grade 10 students at Strathcona-Tweedsmuir, a private school in Okotoks. The school has launched its own investigation and cancelled all field trips until further notice.

Odland said the group was highly skilled and knew what they were doing.

"They assessed, they consulted experts, and they made a choice and went. I'm okay with the process that leads to that. It's a sound one. I'm certainly not OK with the fact seven people ended up dead because of it."

The avalanche area is closed, but it will reopen soon.

In spite of the risks, backcountry skiing is an unregulated sport. No one can shut down a mountain.

Participants say it's part of the joy, to find a mountain where you're the first to ski in the untouched snow.

Even Clair Israelson of the Canadian Avalanche Centre, though clearly mourning the deaths of the students, defended the sport.

"I'm a backcountry skier. There are tens of thousands of British Columbians that go into the mountains because it brings value to our lives. I'd hate to see that privilege taken away from us."

Many say there is a way to make the sport safer. More than 10 years ago, avalanche research in Canada was severely cut by the federal government. They say put more resources back into studying the slides so the warnings are explicit and people believe the danger is real.