NORTH COUNTY -- The smoky pall from a fast-moving brush fire near Palm Springs that killed four firefighters and critically injured another, turned what were expected to be blue skies into a dull reddish-gray Thursday in North County.

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Arson is suspected as the cause of the blaze, dubbed the Esperanza fire, which left the acrid smell of smoke hanging heavy in the air as night fell Thursday.

Two strike teams of firefighters were sent from San Diego County to help fight the fire, which started at 1 a.m. Thursday, including crews from the North County Fire Protection District, Vista, San Marcos, Encinitas and Escondido. Ramona Air Attack also sent two planes -- loaded with 1,200 gallons of fire retardant each -- as well as 10 personnel, an air base manager said.

The firefighters killed and injured were from the U.S. Forest Service and were stationed in Idyllwild. Three died immediately while attempting to protect a home; a fourth died later in the hospital. The surviving firefighter was hospitalized in critical condition with burns over 95 percent of his body.

Their names and information on where they lived were not yet available, though the Idyllwild Town Crier identified one of the deceased as Capt. Mark Loutzenhiser, citing an interview with a fire official.

Authorities are offering a $100,000 reward in an effort to catch the arsonist.

When news of the deaths reached North County firefighters, some of them said they were concerned about who had died and how. In addition to local strike teams working the fire, many firefighters have friends in other agencies.

"We've just basically gotten all our information from the news, and we're pretty much bummed about (the deaths)," said fire engineer Becky Barr at a Valley Center fire station. "We're discussing the things that happened this morning to make sure it doesn't happen here."

Vista fire Capt. John Greenstone said the flag is at half-staff in front of the fire station, and firefighters have placed a black band across their fire badges to show their respect for the firefighters who died and were injured.

"It's a pretty harsh reminder of what the conditions can be," Greenstone said. "It's a reminder that this is dangerous work."

Greenstone said such tragedies show how even experienced firefighters can be harmed when weather conditions shift.

Vista has nine personnel fighting the fire. Greenstone said his department quickly called fire officials in Riverside and determined the Vista firefighters were not injured.

Staff at the California Department of Forestry and Fire Protection headquarters in Rancho San Diego sent out a list of safety precautions for review, and many crews took time to go over it.

Firefighters at Rainbow, Red Mountain, Miller and Deer Springs CDF stations in North County had been doing wildfire drills earlier this week, said Capt. Jeff Lowe at Ramona's fire headquarters station.

As of 8:10 p.m., the Santa Ana wind-whipped flames had consumed about 12,000 acres -- almost 19 square miles -- in the Riverside County area northeast of Palm Springs and was only 5 percent contained, said Robin Prince, a spokeswoman with the Riverside County Fire Department. Firefighters were reckoning with the likelihood of gusts up to 60 mph, officials said, raising the possibility that the fire could spread much farther and wider.

As ash rained down in Southwest Riverside County, smoke grew thicker and more fog-like by mid-day over northern San Diego County.

Authorities with the San Diego County Air Pollution Control District lowered the air-quality level in some areas late Thursday as contaminants threatened to "reach unhealthful levels" in locations with heavy smoke.

Air-quality authorities also warned residents to stay inside and refrain from excessive physical activity in order to avoid health problems associated with the smoke.

Authorities at Palomar Medical Center in Escondido and Tri-City Medical Center in Oceanside said Thursday evening that no one had come into the hospitals with smoke-related complaints. Still, many North County residents expressed concern as the smoke spread over the county.

"The sun is, like, orange," said Judy Armendariz, of San Marcos, about 1 p.m. "It's really weird."

The smoke spread all the way to the coast, and created an orange hue as far west as Encinitas.

Pete Lister said he left his paint job in La Costa early because of the smoky conditions.

"My eyes were burning," he said. "I can't imagine what it's like a little closer to the scene."

Continuing Santa Ana wind conditions will keep smoke hanging over North County for the next day or two, said meteorologist Steve Vanderburg, with the National Weather Service.

However, the offshore flow is expected to "switch around a bit" over the weekend, which should thin the smoke out, he said.

Temperatures are expected to remain high, reaching the 80s inland and on the coast. Winds may reach 50 mph, peaking Friday morning, Vanderburg said. Humidity will remain low, at 5 percent to 10 percent, but temperatures are expected to begin cooling to around the 70s by Sunday.

Meanwhile Thursday, a small grass fire behind Ramona High School was knocked down mid-day, authorities said, before it could be fanned by Santa Ana winds.

The fire, which officials said was suspicious in origin, was reported at 11:15 a.m. in a field behind 1401 Hanson Lane, a spokeswoman said. It burned about 2 acres, she said.

Some students spotted people in the area who fled after the fire started, Ramona fire Capt. Bert Ramirez said.

"We know that it was set, but we don't know if it was intentional or accidental," Ramirez said.

Aircraft immediately responded to the area, but were not needed, he said. The fire was completely out by 2 p.m. although firefighters remained on scene mopping up the area for another hour.

Ramona Unified School District assistant superintendent for human resources, Joe Annicharico, said the fire did not disrupt any classes or scheduled events on campus.

Seventy-four students in the Nuevo Headstart program, which is not part of the district but rents space on the campus, were evacuated as a precaution, he said.

Annicharico said the fire damaged the netting over the varsity baseball field and some undeveloped land that belongs to the district.

- The Associated Press and staff writers Yvette Urrea and Adam Kaye contributed to this report.