Authorities evacuated the canyon Sunday after the fire broke out in Angeles National Forest, an urban recreation area near Los Angeles that is popular with hikers, campers and off-road vehicle enthusiasts.

About 50 residents and 1,000 campers were evacuated, CNN affiliate KABC reported.

"I know we have some neighbors up there that didn't leave. We're really worried about them," evacuee Toni Kova told KABC.

Kova and her husband live in Camp Williams, which was evacuated Sunday.

"We saw the smoke and the fire coming over the hill. The river is literally our front yard and the mountains that the fires were coming over," she said. "We were like, we gotta start packing up."

Officials told KABC the fight was made more difficult by the steep and rugged terrain. The U.S. Forest Service in California said on its Twitter feed it expected a "very active" fire because of low humidity and high temperatures.

Between 10,000 and 12,000 people use the area during an average holiday weekend, according to Inciweb, the government fire-tracking website.

Structures in Camp Williams, a camping and recreational vehicle resort, are in danger from the fire even though it is burning to the northeast toward a wilderness area, Inciweb said.

About 400 firefighters, a heavy air tanker and six helicopters are working on the blaze, which had burned through about 4,000 acres of brush by Monday afternoon, according to Inciweb. The brush, comprised mostly of chaparral, is very thick in some areas.

The fire is 5% contained, the website said.

Inciweb said the cause of the fire was under investigation. CNN affiliate KTLA-TV in Los Angeles, citing a U.S. Forest Service official, reported the fire began when a car caught fire.

The Red Cross has opened an evacuation center in Glendora, California.