More details have been released about a group of back-country ski guides and their 11 clients who were hiking through an avalanche-prone area Tuesday morning as we learned that eight have been found dead and one remains missing.

It's being called the deadliest California avalanche in decades. Deep in Nevada County, in the Castle Peak area north of I-80 and near Donner Summit, nine people lost their lives Tuesday and six survived out of the 15-person group.

The avalanche occurred around 11:30 am, but due to the intense snow Tuesday and the remoteness of the location, rescuers did not reach the group of survivors until 5:30 pm. The six, one guide and five clients, five women and one man, were found sheltering under a tarp, as the Chronicle reports.

The four guides were with Truckee-based Blackbird Mountain Guides, and initial reports suggested they had 12 clients with them. As Bay Area News Group reports, one of the group canceled their trip at the last minute.

The group of skilled rescuers numbered around 50, and their effort to reach the survivors involved specialized snowcats used to traverse hazardous terrain, and then a two-mile ski to the survivors' location. They were able communicate with rescuers using the iPhone's SOS feature.

"Extreme weather conditions, I would say is an understatement. Lots of snow, gale force winds making it impossible to see,” said Nevada County Sheriff Shannan Moon at a Wednesday morning press conference. "It was very much a slow and steady pace."

The back-country ski expedition was in its third day, and the group of 15 had spent two nights in ski huts at Frog Lake. When the avalanche hit, they were hiking back to a trailhead and the group was reportedly somewhat close together at the time. One of the group reportedly called out "Avalanche!" and the group was very quickly overwhelmed.

Sheriff Moon explained that the six survivors were able to recover three of the bodies before rescuers arrived.

As of Wednesday morning, eight of the nine have been found, and all nine were being confirmed deceased. Two of the survivors were reportedly immobile due to injuries when they were found. One remained hospitalized on Wednesday, and the other was treated and released Tuesday night.

The bodies of the deceased remain on the mountain Wednesday due to the difficulties of the recovery effort.

"Due to the ongoing challenges of the weather, the avalanche conditions, the effort remains ongoing as well as our search for the remaining skier," Moon said at the press event.

None of the identities of the victims have been released as of yet.

Moon added that her office was in contact with Blackbird Mountain Guides about the decision to go ahead with the trip despite forecasts showing instense snowfall on the way.

The last deadliest avalanche in the Tahoe area occurred over 40 years ago, when seven people were killed at Alpine Meadows Ski Resort in March 1982.

Placer County Sheriff Wayne Woo also spoke at the news conference Wednesday, as the Chronicle reports, telling all other prospective adventurers to stay out of the area.

"Please avoid the Sierras," Woo said. "Avoid mountain travel … please allow us to focus all of our resources on continuing to recover the bodies."