A wildfire that blackened more than 3,600 acres of Southern California\'s Angeles National Forest forced about 12,000 people to evacuate, officials said. Investigators said it was believed the wildfire started Sunday after a car caught fire. Firefighters struggled the control the fire, which was not considered to be a threat to any buildings, officials said. The evacuees left the popular recreation area Sunday afternoon, Los Angeles television station KTLA reported. More than 200 firefighters were working in an area around Glendora, utilizing an array of air tankers, helicopters, brush engines, water engines and bulldozers. The fire in the 640,000-acre forest was considered 5 percent contained. The blaze began about 3½ miles east of California Highway 39 in the vicinity of Camp Williams Park, near the East Fork of the San Gabriel River, the Los Angeles Times reported. The area was packed with people swimming and picnicking on the Labor Day weekend. Hundreds of people live in trailers and cabins near the park. Authorities ordered the area evacuated because there is only one road in and out. The fire was moving northeast toward the Sheep Mountain Wilderness Area.