Investigators on Monday concluded that the Malibu fire started in the vicinity of a cave in Corral Canyon known for being a late-night party hangout.
Authorities said they still don't know whether the Saturday morning blaze was intentionally set but are convinced that people who were at the cave at that time know what happened.
"We believe there are witnesses who have yet to come forward [and who] can provide crucial information to assist us in this investigation," said L.A. County Sheriff's Department spokesman Steve Whitmore.
Neighbors in Corral Canyon said they were used to parties at the cave on weekend nights, with cars driving up the road into the early morning.
About 2:30 a.m. Saturday, two residents on Corral Canyon Road said they heard a car horn sounding repeatedly as it drove past their enclave.
"I thought it could be drunk people," said Scott Palamar, a multimedia producer. But when he saw the flames on the ridge a couple of hours later, he changed his mind. "My theory is that it was the person who started the fire . . . trying to wake people up."
Across the street, Brooke Halpin had also heard the honking but did not connect it to the fire until a neighbor called at 3:25 a.m. to tell him about the fire.
The arson investigators' conclusion is likely to heighten a debate in Malibu over a proposal to allow more overnight camping in mountain recreation areas.
The fire -- which destroyed 53 homes, 35 outbuildings, 37 vehicles and one mobile home, and damaged 45 structures -- was expected to be fully contained Monday night.
"They're continuing with mop-up operations and will be throughout the evening and into the morning, but everything is looking quite well," said Inspector Frank Garrido, a spokesman for the Los Angeles County Fire Department.
Palamar, the Corral Canyon resident, said he had urged officials to put a gate near the entrance to the cave area to prevent bonfires and partying, but he said the idea was rejected.
In the wake of the blazes, Malibu officials have vowed to impose a requirement for a ranger or other trained individual to monitor the camping areas around the clock. The city is also considering a ban on campfires.
On Dec. 5, the Malibu City Council is expected to vote on a proposal by the Santa Monica Mountains Conservancy that would encourage more use of the mountains by hikers and campers. It would provide for overnight camping at Corral Canyon and Charmlee Wilderness Park, near the Ventura County line.
But residents say allowing overnight camping in the arid hills and canyons would be irresponsible now that fire season runs year-round in Malibu.
At the fire's peak Saturday, about 2,000 firefighters were battling the blaze, but as red flag wind warnings ended, so did the wind-driven fire. By Sunday, hundreds of firefighters were allowed to return home. On Monday, 1,100 firefighters worked on containment.
The shelter at Channel Islands High School in Oxnard is no longer open, but Agoura High School's shelter is still available for those who need a place to stay, Garrido said. Residents were allowed back into their homes, with proper identification, as of 8 p.m. Sunday.
Anyone with information about the incident is asked to call the sheriff's Lost Hills Station at (818) 878-1808 or (310) 456-6652.
Times staff writer Tami Abdollah contributed to this report.