The Angeles National Forest, northeast of Los Angeles in the recreation-rich San Gabriel Canyon, was closed Labor Day as crews continued to battle a wildfire that broke out Sunday.

The Williams Fire tore through the Angeles National Forest on Sunday, forcing authorities to evacuate thousands from the park. By Monday midday, it had grown to 4,000 acres. Popular with campers, hikers and off-road vehicles, the urban recreation area will remain closed for the remainder of the Labor Day weekend.

Officials at Inciweb, a government fire-tracking website, estimate that some 12,000 people use campgrounds in the park on an average holiday weekend. The Angeles National Forest is also home to rehabilitation centers and a private community at Camp Williams Resort above the city of Glendora.

Park officials closed all campgrounds, including those they said were not in the line of fire, to clear the road in and out of the San Gabriel Canyon for fire trucks and emergency vehicles.

On Monday morning, officials set up an evacuation center at nearby Glendora High School, the Los Angeles Times reported. 

Fueled by dry heat and heavy brush, the flames marched uphill toward the Sheep Mountain Wilderness area Monday as some 300 firefighters and four water-dropping helicopters fought back. A DC-10 capable of dropping thousands of gallons of retardant was also sent to battle the blaze.

The fire, which covered about 5.5 square miles on Monday, is about 5 percent contained, according to Inciweb. It has posed no threat to any structures so far and is believed to have broken out between Camp Williams and a Shooting Range on East Fork Road around 2:30 p.m. PDT Sunday afternoon.

The cause remains under investigation, though officials told CBS News station KCAL that it was believed campers barbecuing may be to blame.

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