LOS ANGELES (CBSLA.com) — The Williams Fire burning in the Angeles National Forest was at 25 percent containment Tuesday afternoon, the U.S. Forest Service said.

An estimated 800 firefighters will continue to clear new fire lines and hold existing ones in hopes of expanding containment of the blaze, which has burned approximately 3,600 acres.

The brush fire is also being battled from the air with 11 tankers and 10 helicopters.

The fire is in an area that has not burned in at least 15 years, officials said. Firefighters fear steep terrain and dry fuel could help it spread quickly.

The Williams Fire began around 2 p.m. Sunday in the Camp Williams trailer park, along East Fork Road off of San Gabriel Canyon (39) Road, the U.S. Forest Service said.

Officials have not determined the cause.

Two park rangers and two camp inmates have been injured.

There was no immediate threats to structures, but as a precautionary measure, officials called for evacuations at Williams Restaurant, private camps, an inmate rehabilitation camp and a ranger station.

The American Red Cross has set up an evacuation center for residents at Glendora High School.

The fire is not expected to be fully contained until Sept. 13.