Three ski patrol members who fell to their deaths in a volcanic fissure were probably asphyxiated by poisonous gas spewing from the vent, a coroner said Friday.

After they fell, two of the men could be heard calling for help "and then were silent within a minute or two," said Rusty Gregory, chief executive officer of the ski area on Mammoth Mountain, an 11,053-foot dormant volcano 190 miles east of San Francisco.

Mono County Coroner Ralph Obenberger said the men were asphyxiated by carbon dioxide gas. Autopsies were pending.

The vent releases volcanic gas from deep within the Earth. It is normally surrounded by a plastic fence to keep skiers away, but the fence had been nearly buried by a record 52 feet of snow. The ski patrol went to the site to raise the fence before opening the area.

The snow under the team collapsed, causing two patrol members to fall 21 feet to the rocky bottom of the 6-foot-diameter hole.

One of the dead was a patrol member who died trying to reach the other two. Another patrolman who followed was saved by a colleague who held his breath, jumped in and attached a rescue rope, Gregory said.

The dead were identified as John "Scott" McAndrews, 37, who had been on the patrol one year; James Juarez, 35; a five-year patrol veteran; and Charles Walter Rosenthal, 58, a university researcher and snow expert who had been with the patrol since 1972.

Rosenthal went in first carrying oxygen, trying to get to the others "without regard for his life, probably knowing more than the others about the dangers," Gregory said.

"He apparently was overcome by the variety of toxic gases and the oxygen deprivation that likely occurred," Gregory said.

Ski patrol member Jeff Bridges then went in wearing an oxygen mask, but he also fell unconscious. Another patrol member, Steve McCabe, then held his breath, went in about 15 feet, hooked a rope to Bridges and pulled him out.

"And that's what saved Jeff Bridges' life," Gregory said.

Bridges and six employees who took part in the rescue were admitted to a hospital. All were expected to be released Friday.

On Thursday, the mayor said police had determined carbon monoxide gas was present inside the fissure, but authorities on Friday identified the deadly gas as carbon dioxide.

"We're all shaken by this," said Bob Kittle, a social worker who volunteered to do grief counseling for the ski patrol. "Anyone who has skied Mammoth Mountain has skied right by this."

Gregory offered to close the 3,500-acre ski area Friday if patrol members did not want to work, but the mountain reopened on schedule.

"This is a community that cares deeply about its people, that comes together at times like this," he said.