Emergency crews have recovered the bodies of nine skiers killed in a California avalanche. (AP: Godofredo A. Vásquez)

The bodies of nine back-country skiers killed in a California avalanche four days earlier have been recovered, following a harrowing operation hindered by intense snowfall.

On Saturday morning, local time, a search team reached the bodies of eight victims and found one other who had been missing and presumed dead since Tuesday’s avalanche on Castle Peak near Lake Tahoe.

The ninth person who was missing was found “relatively close” to the other victims, but it was impossible to see them because there were white-out conditions on Tuesday when the others were located.

The recovery efforts had been put on hold for several days because of heavy snow and the threat of more avalanches.

Helicopters with the California National Guard and California Highway Patrol recovered the bodies by hoisting them from the mountain and bringing them to nearby snowcats — trucks outfitted for transportation on the snow.

Officials said on Friday they were using water to break up the snow in the area as avalanche mitigation work, a technique designed to intentionally release unstable snowpack to reduce the risk when rescue crews go in.

Three guides among those killed

The skiers were part of a group of 15 people on a three-day trek in Northern California’s Sierra Nevada.

Three of the guides — Andrew Alissandratos, 34, Nicole Choo, 42, and Michael Henry, 30 — were among those killed, officials said.

According to biographies on the company’s website, Mr Alissandratos was originally from Tampa, Florida, and moved to Tahoe about a decade ago.

He enjoyed a wide array of adventure activities, from back-country exploration to rock climbing.

Mr Henry moved to Colorado in 2016 and then to Truckee three years later. He was described as “laid back” and devoted to sharing his knowledge and love of the mountains with others.

There was no bio for Ms Choo on the website.

Recovery of the missing skiers was held up by heavy snowfall and the risk of further avalanches. (AP: Godofredo A. Vásquez)

“This was an enormous tragedy, and the saddest event our team has ever experienced,” Blackbird Mountain founder Zeb Blais said on Wednesday in a statement.

“We are doing what we can to support the families who lost so much, and the members of our team who lost treasured friends and colleagues,” he said.

The six other skiers killed were women and part of a close-knit group of friends.

They were identified as Carrie Atkin, Liz Clabaugh, Danielle Keatley, Kate Morse, Caroline Sekar and Kate Vitt, all in their 40s. They lived in the San Francisco Bay Area, Idaho and the Lake Tahoe area.

They were experienced back-country skiers and knew how to navigate the Sierra Nevada wilderness, their families said.

“We are devastated beyond words,” the families said in a statement.

“They were all mothers, wives and friends, all of whom connected through the love of the outdoors.”

Two of the friends got out alive and were rescued along with four others, including one guide, after Tuesday’s avalanche. Their names have not been released.

Stuck on the mountain for hours

The avalanche struck on the last day of the skiers’ three-day tour, when the group decided to end the trip early to avoid the approaching snowstorm.

At about 11:30am on Tuesday, the six survivors called for help, describing a sudden and terrifying slide that was approximately the length of a football field.

In the aftermath they discovered the bodies of three fellow skiers, according to police.

The terrain is closed to visitors while the investigation is underway. (AP: Stephen Lam/San Francisco Chronicle)

Rescuers were not able to reach them until about six hours after the initial call for help. They found five other bodies, leaving only one person unaccounted-for.

But it was immediately clear to rescuers that it was too dangerous to extract the bodies at that time due to the heavy snowfall and threat of more avalanches. Those conditions persisted on Wednesday and Thursday.

Nevada County Sheriff Shannan Moon said investigators were looking into the group’s decision to proceed with the trip on Sunday despite the forecast for a major storm.

It is not known if the guides were aware of an increased avalanche warning as they set out on Tuesday.

Before the disaster, Blackbird Mountain Guides, the tour company, said its guides were highly skilled in dealing with extreme conditions.

Blackbird Mountain Guides said in a statement on Tuesday, local time, it was coordinating with authorities to help with the search and rescue mission.

The investigation to determine if the guides were criminally negligent surprised legal experts who said the probe had scant precedent.

Shannan Moon (centre) says investigators were looking into why the group went despite the weather warnings. (AP: Tran Nguyen)

Back-country tour clients typically sign waivers of liability that shield guides and their companies from potential lawsuits, according to attorney Andrew McDevitt.

That means recreational skiers assume certain risks, such as potential collisions with rocks or trees and changing snow conditions.

But Utah ski injury attorney, Rob Miner, said the waivers did not absolve the guide companies of responsibility if they did not follow appropriate safety protocols or failed to fully inform their clients of the risks.

“They’re guides. It is presumed they will not guide you into an experience that may kill you, or that creates an unreasonable risk of death,” Mr Miner said.

The Nevada County Sheriff’s Office declined to share more information when it announced the investigation on Friday.

A state agency that regulates workplace safety also said it opened an investigation into the tragedy.

The area near Donner Summit is one of the snowiest places in the Western Hemisphere and was closed to the public until just a few years ago.

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.