A wind-whipped wildfire started by an arsonist killed four firefighters and stranded up to 400 people in a campground when flames burned to the edge of the only road out, officials said.
The Forest Service crew was trying to protect a house on Thursday as dry desert winds of 40kph or more blew a wall of flames down on them in the hills northwest of Palm Springs.
"They had left their truck to do structure protection when the fire overran them," said Forest Service spokesman Pat Boss, adding that the flames came down so quickly they had no time to retreat to their engine or use protective sheltering.
Fire officials said the blaze was deliberately set around 1am on Thursday. Fire Chief John Hawkins said the arson "constitutes murder."
It was the deadliest wildfire firefighting disaster in the US in five years.
Thursday's fire left 400 campers at the Silent Valley Club park stranded for hours but in no immediate danger, said Charles Van Brunt, a park ranger.
"Everybody is hunkered down here. They're fighting the fire around us. It's across the street from us," Van Brunt said.
Three of the firefighters killed on Thursday died at the scene, and two were hospitalized in critical condition.
One of those two died several hours later. The other had burns over 95 percent of his body, Boss said.
The surviving firefighter had severe respiratory damage, said Dev Gnanadev, a trauma surgeon at Arrowhead Regional Medical Center.
Homicide detectives were sent to the scene to work with FBI agents during the investigation. One official believed the blaze was set just as the winds picked up in order to maximize destruction.
Authorities planned to offer a US$100,000 reward in the case.
"Turn that scum in, please," Riverside County Supervisor Marion Ashley said.
In less than 24 hours, the fire blackened almost 97km2, and more than 1,100 firefighters were brought in along with water-dropping helicopters and planes. Ten structures were destroyed, and at least five were homes.
The fire, which was only 5 percent contained, roared 24km to the west from where it began.