AMNA NAWAZ: We start the day's other headlines, meanwhile, in Northern California.

At least eight people are dead following an avalanche that tore through the Sierra Nevada, and, tonight, search-and-rescue teams are scouring the mountain wilderness for one remaining body.

Despite an urgent search-and-rescue effort overnight, tragedy in the Northern California mountains.

Of the nine missing backcountry skiers caught in an avalanche yesterday morning, nearly all of them have been found dead.

It is the deadliest avalanche to occur in the United States in more than four decades.

At a press conference this afternoon, the Nevada County sheriff said the final missing person likely perished too.

SHANNAN MOON, Nevada County, California, Sheriff: We did have a conversation with the families of the folks that are still outstanding and let them know that our mission has went from a rescue to a recovery.

It's a difficult conversation to have with loved ones, so I please ask for your patience.

AMNA NAWAZ: The group of 15 skiers in all, including four guides, were wrapping up a three-day guided backcountry tour and heading back to the trailhead when the avalanche struck at Frog Lake in the Castle Peak area northwest of Lake Tahoe.

The dispatch calls came in late yesterday morning.

EMERGENCY DISPATCH: Medical for an avalanche in the area of Castle Peak, reported as nine to 10 people buried, three others attempting to dig them out.

AMNA NAWAZ: Six skiers who were initially rescued huddled under a tarp for close to 12 hours before rescuers could reach them.

ETHAN GREENE, Director, Colorado Avalanche Information Center: When someone's buried in an avalanche, time is of the essence.

You really need to get that person out of the snow and back onto the snow surface as fast as possible.

AMNA NAWAZ: Ethan Greene is the director of the Colorado Avalanche Information Center.

ETHAN GREENE: Events like this one are quite unusual, where you have this many people involved in an accident.

And it sounds like the avalanche conditions in this area were both quite dangerous and also fairly well described and predicted by the Sierra Avalanche Center.

AMNA NAWAZ: A powerful winter storm has buried parts of California in several feet of snow since Sunday.

And gale-force winds caused whiteout conditions that slowed rescue efforts.

MAN: We're walking along the same track here.

AMNA NAWAZ: Video posted by the tour company Blackbird Mountain Guides over the weekend appeared to show that they were aware of the risk of avalanches.

The snowstorm had been in the forecast for nearly a week.

And there have been other harrowing avalanches overseas.

This one struck a moving train yesterday near the Swiss town of Zermatt.

And another avalanche likely derailed this train near the Swiss town of Goppenstein, causing several injuries.

But elsewhere across Europe, avalanches have taken dozens of lives this winter, including 10 in the Italian mountains just in the last week alone.