Officials said the fire appears of suspicious origin because it started in three different areas.
Arson investigators are checking reports of two men with long hair seen leaving the area just as the fire started.
The flames and smoke affected nearby traffic, with the California Highway Patrol shutting down the nearby freeway for more than an hour Wednesday. As conditions improved, more lanes were reopened, with all lanes open by Thursday afternoon.
Los Angeles County Fire Department Inspector Steve Zermeno said about 300 firefighters from the Los Angeles County Fire Department, prison crews and the U.S. Forest Service were called in to fight the blaze.
City News Service contributed to this report.