POPPET FLAT, Calif. -- A Riverside County Fire Department spokesman told The Associated Press on Thursday that fire officials believe that a wildfire near Palm Springs, Calif., was deliberately set overnight.

The spokesman said investigators think the Esperanza fire was started at about 1 a.m. Thursday; a $100,000 reward for information leading to the person responsible has been offered. Riverside County Fire Chief John Hawkins said the arson "constitutes murder."

Earlier Thursday, the U.S. Forest Service said four firefighters were killed and one critically injured while battling the wildfire. Forest Service officials said the five firefighters were trying to protect a home and were inside their engine at about 6 a.m. when it was overrun by flames that were at least 100 feet high.

Three firefighters died at the scene, and two were hospitalized with critical burns. One of those two died several hours later. The fifth had burns over 95 percent of his body. A doctor said his chance of recovery is low.

In response to the tragedy, a spokesman said all U.S. Forest Service personnel have been pulled off the fire to let them "gather their thoughts" and "say their prayers."

Meanwhile, officials said up to 400 people were trapped at a recreational vehicle park near Palm Springs, where the Esperanza wildfire has blocked the only road out.

Highway 243 was blocked in front of the Silent Valley Club, and television station footage showed some vehicles racing through the highway in the midst of flames and smoke just before firefighters closed the road. A ranger near the park said residents were in no immediate danger, and authorities asked them to stay in the area so that firefighters could use the highway in their effort to fight the blaze.

The fire, which started near the community of Cabazon, grew overnight Thursday. Fire Capt. Rick Griggs said wind might limit the degree to which the fire can be attacked from the air.

"(The wind) makes it very difficult for our fixed-wing aircraft ... and helicopters," Griggs said. "We may not be able to fly into some of these areas and not make as aggressive of an attack as we could."

The fire has quickly burned more than 10,000 acres and has destroyed at least five homes since it began. At least one resident suffered minor burns, KNBC-TV in Los Angeles reported.

Fanned by hot Santa Ana winds, the fire has chased about 700 people from their homes near Cabazon. The fire was in a valley that has only a few scattered ranch houses, but evacuations were ordered for the communities of Twin Pines and Poppet Ranch, as well as a juvenile center.

A man who fled his hilltop home with his dogs called the fire spectacular.

"A terrible view," he said, "but spectacular."

The fire was burning across ground cover mainly of grass and chaparral that, according to firefighters, burns hot and fast but without much staying power. The big concern is that it could reach part of the San Bernardino National Forest, which is filled with dead trees from a bark-beetle infestation.

Cabazon is about 90 miles east of Los Angeles and 17 miles northwest of Palm Springs in Riverside County.

Farther north in the wine country of Napa, another brush fire was partially contained after consuming about 80 acres. Strong winds and rugged terrain made the blaze tough to fight, firefighters said.

No homes were threatened, but about two dozen residents were evacuated overnight as a precaution.