Three members of a ski patrol team died Thursday when they fell into a volcanic fissure at the Mammoth Mountain resort, officials said.

Whether they were killed by the 21-foot fall or were also affected by gases seeping from the cavity was not immediately clear.

The victims were part of a four-man team inspecting the mountain after heavy snowstorms and fencing off the gap in the rock, officials said.

Mammoth Lakes Mayor Rick Wood said heat from hot rocks had hollowed out the snow and two ski patrol members fell into the fissure on the 11,053-foot peak in the Eastern Sierra.

A third patrol member attempted a rescue and perished as well, and the fourth was injured, he said.

Additional rescue efforts were conducted by other ski patrol members and local firefighters and paramedics. Resort spokeswoman Joani Lynch said several rescuers were overcome by gas and suffered minor injuries.

The role that gas might have played in the three deaths was uncertain, but the mayor said a police detective told him that "the level of carbon monoxide inside this cavity was extremely high."

None of the victims' names were immediately released.

The mountain, about a six-hour drive north of Los Angeles, is popular with skiers from Southern California. The peak towers over a dramatic landscape in a volcanically active region.

The region has been quiet of volcanic activity for six years, said Dave Hill, a scientist with the U.S. Geological Survey in Menlo Park.

The accident was not related to any volcanic activity, he said.