CAL FIRE says the Avila Fire, which burned about 445 acres and threatened the community of Shell Beach in June, was human-caused and intentionally set.
The fire broke out on the afternoon of June 15, near the Avila Beach Drive off-ramp of northbound Highway 101.
Fire officials say investigators have determined the fire was started in a creek bed along Avila Beach Drive, about 70 feet from the roadway.
They say they have developed a list of possible suspects and the investigation is ongoing.
Hundreds of homes in Shell Beach were threatened by the fire but just one structure sustained damage. Fire officials attributed that to a quick response by fire crews and good defensible space around the threatened properties.
Anyone with information about the fire is asked to contact the CAL FIRE Hotline at (800) 468-4408.