Authorities are investigating whether any criminal negligence was involved in the deadly avalanche that swept California’s Lake Tahoe this week, which killed at least eight skiers and their guides while returning from a three-day backcountry skiing trip.

The Nevada county sheriff’s office said on Friday said that they notified the Occupational Safety and Health Administration (Osha), which regulates workplace safety, of the active investigation.

“We are investigating the incident to determine if there were any factors that would be considered criminal negligence,” said a spokesperson for the sheriff’s office. “It is too early to know if criminal charges will be applicable, as the investigation is preliminary and remains active and ongoing. The district attorney’s office would ultimately be the determining authority for any criminal charges to be filed.”

The avalanche, the deadliest in California’s modern history, killed at least eight people and a ninth skier is still missing but is presumed dead.

Blackbird Mountain Guides, which employed the ski guides that led the trek, said in a statement released on Wednesday that three of their workers died after the avalanche, and one of them survived.

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

“There is still a lot that we’re learning about what happened. It’s too soon to draw conclusions, but investigations are underway,” Blais added in Wednesday’s statement.

As the 15 skiers were coming back from their trip, which started on Sunday, avalanche warnings were already in place. Blackbird Mountain Guides seemed to be aware of the hazardous weather conditions, according to a social media video posted on Sunday.

During a press conference on Wednesday, Shannan Moon, Nevada county’s sheriff, said that the investigation includes learning why the ski trip was not cancelled.

“Those are the decisions that the guide company clearly had made,” Moon said. “We’re still in conversation with them on the decision factors that they made.”

Six of the 15 skiers survived and were rescued, including the fourth guide. Amid hazardous weather conditions, the avalanche victims have not yet been extracted from the mountain.

“Adverse weather, coupled with ongoing avalanche danger, has hindered the recovery process,” a statement released on Friday by the Nevada county sheriff’s office said. A statement released on Thursday said “until the recovery mission is completed, the Nevada County Sheriff’s Office is unable to confirm the identifications, ages, affiliations, or cities of origin for the victims.”

Family members have confirmed the identities of six of the eight people who died in Tuesday’s avalanche. The identified victims – Carrie Atkin, Liz Clabaugh, Danielle Keatley, Kate Morse, Caroline Sekar and Kate Vitt – were part of a group which frequently went on ski trips together.