Angeles National Forest, located outside of Los Angeles, Calif., in the recreation-rich San Gabriel Canyon, was closed Labor Day as crews battled a wildfire that broke out Sunday.

The 3,600-acre Williams Fire tore through the Angeles National Forest on Sunday, forcing authorities to evacuate thousands from the park. Popular with campers, hikers and off-road vehicles, the urban recreation area will remain closed for the remainder of the three-day Labor Day weekend.

Officials at Inciweb, a government fire-tracking website, estimate that some 12,000 people use campgrounds in the park during the average holiday weekend. 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, so that the road in and out of the San Gabriel Canyon could be available for fire trucks and emergency vehicles.

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 battled the fire. 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 and is believed to have broken out between Camp Williams and a Shooting Range on East Fork Road at about 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.