Authorities say they’re working to find out why a group of skiers and their guides continued with their trip after a first avalanche watch was issued on Sunday, two days before an avalanche killed some of them near Lake Tahoe, California.

“Lots of forecasts on this storm. Those are the decisions that the guide company had clearly made. We’re still in conversations with them on the decision factors,” Nevada County Sheriff Shannan Moon said.

Eight skiers were found dead in the Sierra Nevada and a ninth was still missing, officials on Wednesday. Search teams rescued six skiers who reportedly sought refuge in trees following the avalanche that happened on Tuesday.

The group of 11 skiers and four guides was returning from a three-day backcountry ski trip during a winter storm that dumped up to 7 feet of snow and lashed the mountains with 60 mph winds.  

On Sunday, the Sierra Avalanche Center issued an avalanche watch saying “very dangerous avalanche conditions could occur in the backcountry” in a region including the greater Lake Tahoe area.

“HIGH avalanche danger is anticipated in the backcountry Monday evening, lasting through Tuesday night, and possibly into Wednesday,” it said.

An avalanche warning was then issued on the morning of Feb. 17, the day of the incident, extending to Friday. The advisory stated that “HIGH avalanche danger exists in the backcountry” and said “large” avalanches were expected across backcountry terrain.

Moon said a football field-sized slab of snow slid off a mountain in the Castle Peak area, which is about 10 miles north of Lake Tahoe – swallowing the skiers.

“Someone saw the avalanche, yelled ‘avalanche!’ and it overtook them rather quickly,” Capt. Russel Greene of the Nevada County Sheriff’s Office said.

Rescue crews have faced treacherous conditions, including blinding snow and high winds. 

The Nevada County Sheriff’s Office said they have to wait on the weather to recover the victims and that it’s still too dangerous with a continued avalanche threat.