About 500 firefighters worked to douse the blaze, which has burned 4,100 acres since it began Sunday afternoon and was only 5 percent contained Monday night.
The fire was pushing northeast toward the Sheep Mountain Wilderness area. Nine aircraft were assisting firefighters.
Fire officials said the biggest obstacle for battling the blaze is the rugged terrain. They said full containment of the fire was not expected until next Monday.
The Red Cross has established a shelter for those affected by the fire at Glendora High School, located at 1600 East Foothill Boulevard in Glendora.
Investigators are looking into a vehicle that was found burning in the area as a potential starting point of the fire.
"We have our fire investigators from forest service and L.A. County Sheriff's are looking at the investigation of this fire. There was vehicle within the area, we don't know if that had anything to do with the fire," said Nathan Judy, an official with the Angeles National Forest.
"The reason we're at 5 percent containage is because these mountains are so rugged and steep. You can go from a 40- to an 80-degree incline on these mountains, so getting our folks on the ground, putting a line around this fire is going to take a little while," said Judy.
The fire, burning near Highway 39 at East Fork Road on the north end of the San Gabriel Reservoir, was first reported about 2:30 p.m. Sunday and had charred more than 700 acres by 5 p.m. By 9:30 p.m., it was up to 3,600 acres and 5 percent contained.
About 50 residents and 1,000 campers were evacuated from three campgrounds, including Camp Williams and the River Community rehab treatment center, officials said.
Officials said power lines were threatened. At least one power line fell and another was seen on fire.
Highway 39 was shut down from Crystal Lake Road down to the 210 Freeway, officials said. It was to remain closed through the Labor Day holiday weekend.
The cause of the fire, named the Williams Fire, was not known.
The area had not burned since 2003.
No injuries have been reported. No structures were threatened.
