When firefighters arrived on the scene, they immediately called a second alarm. The fire was inaccessible by engines and burning away from the road, said Jim Tomaselli of the U.S. Forest Service.
About 700 fire personnel battled the blaze as well as more than a dozen aircraft.
L.A. County fire Chief Daryl Osby said his department was fully staffed because of the "dry vegetation and summerlike weather conditions we've had."
That staffing allowed more resources to be deployed quickly as the fire grew.Glendora police Chief Tim Staab said one of the suspects admitted starting the fire and was "apologetic."
The area is not a designated camping spot and access to the area is restricted because of fire concerns.The city of Glendora, meanwhile, declared a state of emergency and activated the city’s emergency operations center.
Mayor Joseph Santoro thanked the firefighters and law enforcement involved in the effort.
"This morning at about 6 a.m. it looked pretty terrible out there ... it was a very scary situation," he said.The Glendora Unified School District canceled classes at La Fetra, Cullen, Goddard, Sandburg and Sellers schools and urged parents to pick up students, according to the district's website.
Citrus College in Glendora also canceled classes.
Southern California Edison reported that dozens of people were without power due to the fire Thursday morning.
Scott Sukup of the National Weather Service in Oxnard said conditions were ripe for fire because “obviously we still have the dry and windy conditions going on.”
A red flag warning was extended for that area and other parts of Los Angeles County through 3 p.m. Friday, and Sukup said gusts in the mountains wre expected to be 40 to 50 miles per hour with single-digit humidity.
Winds topping 20 mph were a concern earlier in the day, Miller said, but had died down.
“We still have topography, which is a major factor, a critically dry fuel bed,” he said. “The moisture out here is extremely low, as it is throughout all of L.A. County. Those are our biggest challenges right now.”
Residents from the San Gabriel Valley to the Pomona Valley were also warned to stay indoors as smoke from the fire spread over a large area. The smoke plume was visible from as far away as Orange County.
Residents who can see or smell the smoke should stay indoors and avoid vigorous exercise, said South Coast Air Quality Management District spokesman Sam Atwood.
One of the structures destroyed by the fire was a guest house at the historic Singer Mansion. Ron Galloway, 63, stood in front of the smoldering remains where he had lived for four years. Spanish-style arches were all that remained standing in front of piles of broken roof tiles and the smoldering rubble of the property.
“I lost everything, how am I going to survive?”
The historic landmark was built by the heirs of the Singer family, of the Singer sewing machine company, on a 5.7-acre estate, according to the mansion’s website. At least 11 people rent rooms in two adjacent guest houses.
Galloway said that when he awoke Thursday at 6 a.m. as usual he noticed an orange glow coming from the hillside across from the estate. He quickly ran inside to wake his roommates.
A second brush fire that started late Thursday morning near the area of Pyramid Lake and Hungry Valley -- sparked by a mobile home that was fully engulfed in y flames -- was held by fire crews to no more than two acres and was declared knocked down shortly after 12:30 p.m. No resources were used from the Colby fire.
Staff writers Ari Bloomekatz, Jason Wells, Joseph Serna and Matt Stevens contributed to this report.