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Helicopters and air tankers fought the fire until dusk, Lugo said.
With relative humidity levels of about 12 percent and hot daytime temperatures, the fire fed on dry vegetation on both sides of Highway 39 and sent 40-foot flames racing up San Gabriel Canyon.
Picnickers, campers and hikers were evacuated, including about 15 Boy Scouts who rode a sheriff's department helicopter out of the canyon, said Sheriff's Sgt. David Infante.
The fire was 10 percent contained Tuesday night. No structures were threatened, and the flames were moving away from populated areas, Lugo said.
The National Weather Service has issued a red flag warning of high fire danger for mountain areas in Los Angeles, Ventura, Santa Barbara and San Luis Obispo counties. The warning is scheduled to go into effect at 6 a.m. Wednesday and last until 9 p.m. Friday. Temperatures could exceed 100 degrees in some areas, with relative humidity expected to fall below 10 percent, the weather service said.