Eight back-country skiers have been found dead and one is missing after an avalanche near Lake Tahoe in California, officials say.
The skiers were part of a group of sixteen who were on a three-day trek in Northern California's Sierra Nevada when a winter storm pummelled the West Coast.
It is the deadliest avalanche in the US in more than four decades.
Here's how it unfolded.
Weather warnings issued
On Sunday, local time, a group of 15 skiers made their way up to remote huts situated at 3,415 metres in Tahoe National Forest in California, on Sunday, local time.
The group was on a three-day trek in Northern California's Sierra Nevada.
On the same day at 6:49am, local time, the Sierra Avalanche Center issued an avalanche watch for the area, indicating that large slides were likely to come in the next 24 to 48 hours.
Two days later, on Tuesday, local time, as the group was making its way back out to a trail head, the centre increased the watch to a warning, an avalanche hit near Castle Peak, trapping the skiers.
Search-and-rescue crews were dispatched to the Castle Peak area of the Sierra Nevada after a 911 call reporting the avalanche had buried 15 skiers in the snow.
Six of the skiers in the group were found alive.
Spokesperson for the Nevada County Sheriff's Office Ashley Quadros said two of those rescued after several hours of searching were taken to a hospital for treatment.
Heavy snow and the threat of additional avalanches slowed the rescue effort in the mountains near Castle Peak, north-west of Lake Tahoe.
The area near Donner Summit is one of the snowiest places in the Western Hemisphere and until just a few years ago was closed to the public.
Crews faced treacherous conditions in their search for the missing skiers after the avalanche struck on Tuesday morning, local time.
According to AP, as eight people were found dead, it is the deadliest avalanche in the US since 1982.
The avalanche in 1982 crashed into the Alpine Meadows Ski Resort near Lake Tahoe, killing seven people, including four resort employees who remained on site even though the resort had been closed for the day because of the dangerous conditions.
A 22-year-old chairlift operator was rescued five days later after a specially trained dog found her in the remains of the ski chalet.
The deadliest avalanche recorded in the US was in 1910 in Wellington, Washington, where 96 people were killed after a massive wall of snow swept two Great Northern passenger trains into a gorge.
Investigation launched into why group proceeded despite storm
Nevada County Sheriff Shannan Moon said investigators were looking into the decision to proceed with the trip on Sunday despite the forecast for a major storm.
"We're still in conversation with them on the decision factors that they made," Ms Moon said at a news conference on Wednesday, local time, when announcing the deaths of the skiers.
Before the disaster, Blackbird Mountain Guides, the tour company, said its guides were highly skilled in dealing with extreme conditions and that it offered avalanche education.
It's not known if the guides would have known about the avalanche warning as they returned to the trail head.
"If you've booked the Frog Lake Huts in Truckee, trust our guides to elevate your trip to the next level," Blackbird Mountain Guides said on its website, advertising the trip.
"We'll navigate in and out of the huts, manage the risks, and find the best terrain and snow quality for you and your group!"
Blackbird Mountain Guides said in a statement on Tuesday, local time, it was coordinating with authorities to help with the search and rescue mission.
Three of the guides were among those killed, officials said.
"Our thoughts are with the missing individuals, their families, and first responders in the field," Blackbird said in a statement on Wednesday, local time.
What is an avalanche and what causes one?
An avalanche is a rapid flow of snow down an inclined slope, such as a mountainside or even the roof of a building, according to the Bureau of Meteorology (BOM).
In North America, avalanches are measured using a size scale of one to five, with one being not deep enough to bury a person, to five, a snow avalanche that could bury or destroy a village.
Avalanches in Australia are usually a one or two using the North American size scale.
"This is mostly due to the amount of snow we get, which typically reaches one to two metres in depth," BOM said on its website.
According to BOM, avalanches can be caused by natural forces such as the pull of gravity on a steep slope or an increase in temperatures.
It said they could also be caused by human activity, such as the load of a skier across the snow, or explosives as part of avalanche control.
They can occur when one layer of snow slides off another, which is called a surface avalanche or the entire snow cover slides on the ground, which is known as full-depth avalanche.
Other avalanches around the world
Avalanches have claimed an average of 27 lives each winter in the United States over the past decade, according to the Colorado Avalanche Information Center.
More than 150 people die on average worldwide each year from avalanches, according to the US National Weather Service.
There have already been several fatal avalanches around the world this year.
Two skiers were killed on Sunday local time in Italy and another was in serious condition after an avalanche struck a slope near Courmayeur, Italy's alpine rescue said.
The avalanche hit a stretch of the Canale dei Vesses, known for steep passages popular with off-piste skiers, west of Courmayeur in the Val Veny area at the foot of the Mont Blanc.
In France, two French off-piste skiers died after being buried by an avalanche at Val-d'Isere in January, the resort's tourism office said. The two men were buried under 2.5 metres of snow.
In Japan, continued snowfall since late January has left dozens of people dead and injured hundreds more.
Many fatal accidents happened when mounds of snow fell on residents from roofs, or people tumbled while trying to clear it, according to police and local officials.
ABC/AP
