The brush fire that raced up Sepulveda Pass and into ritzy Bel Air last week, stopping traffic on the 405 freeway and destroying six homes, was started by an illegal cooking fire, investigators with the Los Angeles Fire Department have concluded.

The Skirball Fire was ignited at an encampment in the brush along Sepulveda Boulevard, where the street passes under Interstate 405, LAFD officials said in a statement Tuesday, even as dozens of firefighters continued working to fence in the flames.

The fire has blackened more than 400 acres since it broke out nearly a week ago. It was 85 percent contained Tuesday.

Los Angeles City Councilman Mike Bonin said homeless people who were being moved around by increasingly restrictive city policies were making improvised homes in the city’s fire-vulnerable canyons.

“We can't continue to allow encampments on our sidewalks. We can't go let them in our parks and the canyons,” Bonin said. “We keep moving encampments around from place to place.”

Bonin said more such fires could occur, unless the city takes additional steps to reduce homelessness in the canyons.

“We need to make sure we're taking every step possible to reduce all those risks, whether it's regulating the equipment that is used to clear brush or doing everything we can to address homelessness,” Bonin said. “But we cannot have people living in encampments and lighting fires in a high-fire-danger area, because we can wind up with thousands of people dead in a very short time.”

Canyons and other areas where homeless people set up encampments are surveyed each year by the Los Angeles Fire Department in advance of the fall and winter rainy season so that the information can be shared in times of potential floods and mudslides with law enforcement and other emergency workers.

Such areas had not previously been surveyed in June and July, in advance of L.A.’s fire season, said LAFD spokesman Capt. Erik Scott.

The encampment where the Skirball Fire started was not on the endangered encampment areas survey, he said. However, such areas would be surveyed next year in advance of both fire and storm seasons.

No one was present when firefighters first arrived on scene last Wednesday, and no arrests have been made, according to the statement.