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RIVERSIDE, California (CNN) -- A raging wildfire near Palm Springs that has killed four firefighters and burned about 10,000 acres was deliberately set, said John Hawkins, of the California Department of Forestry and Fire Prevention's Riverside County Fire Department.
"This is an arson fire. This is a deliberately set arson fire," Hawkins told reporters Thursday, adding that the crime constituted "murder."
A $100,000 reward was established for information leading to the alleged fire starter. (Watch firefighting efforts -- 1:35)
The four firefighters killed were part of a Forest Service crew trying to protect a house as dry desert winds of 25 mph 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 the flames came down so quickly they had no time to retreat to their engine truck or use protective sheltering.
Three firefighters 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 fifth firefighter is in critical condition, said Hawkins.
As many as 700 people remained Thursday night in a recreational vehicle park less than a mile from the blaze. The sky above the ridges of Silent Park Valley is "orange," said Charles Van Brunt, who was the park ranger managing the private members-only Silent Valley Club Thursday night. But Van Brunt said no one has been forced to leave the park, which serves as the only evacuation route out of the neighboring town of Poppet Flat.
A ranger alerted residents around 3 a.m. Thursday about the blaze. Some left, but no one kept track of how many, Van Brunt said. By nightfall, firefighters had shut down the road out of the park as a precaution, Van Brunt said. He said he wasn't worried, because winds are blowing the blaze away from the park.
"We're not trapped. We're very safe here," Van Brunt told CNN. "The roads are closed but that's just primarily so the firefighters can do their job. But on the ridges around us, we can see the flames. It's safer for us to be here than trying to get everyone out right now."
Van Brunt said hundreds of people typically converge on the RV park to celebrate Halloween with a hayride and trick-or-treating.
He said everyone in the park is "very sad" for the deaths of the firefighters.
"These people were heroes"
"These people were heroes, and they did what they do every single day," said Allison Stewart, a district ranger for the San Bernardino National Forest, as officials begin the painful process of notifying relatives.
"And they did it with no reservations, and they did it for the love of what they do," she said.
Hawkins did not provide details about how officials knew the fire was deliberately set, citing the ongoing investigation. However, Hawkins and other officials said that the blaze was set in a place and under conditions that would likely guarantee maximum damage. "It was set in alignment with the wind, the slope," Hawkins said. "It was basically set to go."
Arson investigators from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were on the scene.
Local, state and federal officials expressed outrage as they talked to reporters. Members of Congress representing the area were spearheading a drive to raise money for the slain firefighters' families.
California Gov. Arnold Schwarzenegger issued a statement lamenting the death of the firefighters.
"In attempting to save residents' homes and lives, they sacrificed their own," he said. "Our hearts go out to their families during this difficult time."
Winds gusting up to 30 mph
The fire grew exponentially throughout the day. As of 3 p.m. (6 p.m. ET), Hawkins said he would estimate it had burned at least 10,000 acres. Asked about containment, he said he would guess it was only 10 to 15 percent contained.
One civilian burn injury was reported, he said, but he did not provide details.
Winds in the area of the fire were erratic, California Department of Forestry and Fire Prevention Capt. Don Camp told CNN.
"Winds are sustained at 18 to 25 mph with gusts up to 30 out of the east, which is ... making a major push on the fire to the southwest," he said.
Many structures have been destroyed, including homes, according to what can be seen on video.
Hawkins confirmed that homes had been destroyed, but said he did not know how many.
Patrick Chandler of the Riverside County Fire Department said the area had been under a red flag warning for about two days because of dry conditions and gusty winds.
The Riverside County Fire Department is a unit of the California Department of Forestry and Fire Prevention.
CNN's Ashley Fantz contributed to this report.
Copyright 2006 CNN. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed. Associated Press contributed to this report.
A wildfire in the Bitterroot National Forest in the State of Montana.
A wildfire has been reported near Palm Springs, California. 

At least 4 United States Forest Service firefighters were killed and 1 seriously injured, with burns on nearly 95% of his body.  The firefighters were overcome by fire while attempting to put out a blaze inside a home. The fire has burned nearly 10,000 acres and has destroyed 5 houses including 5 other buildings.

"We had a lot of fire activity overnight. Normally the fire dies down and the winds let up but that didn't happen last night. It burned like it was daytime," said California Department of Forestry spokesman, Bill Peters.

"The chances of survival are low when you have that bad of an injury," said Arrowhead Regional Medical Centre surgeon, Doctor Dev Gnanadev who also added that the firefighter has suffered "severe respiratory damage."

Witnesses have reported that 2 males were seen leaving the area of the fire at 1:00 a.m. on Thursday morning.

Authorities have determined that the blaze was due to arson and are offering a $500,000 USD reward for information leading to an arrest, but no information on how or why the fire was set has been released. Authorities say that murder could be among the arson charges who whoever is responsible.

"This is an arson fire. This is a deliberately set arson fire," said California Department of Forestry and Fire Prevention, John Hawkins.

Nearly 1,000 people have been evacuated from the area.