ANAHEIM, Calif. — A destructive Southern California wildfire this fall was ignited by windblown embers from an oak that was still smoldering after an earlier fire that is believed to have started when a vehicle struck a highway flare, authorities said Monday.

The first fire damaged four homes and another structure in late September. The second fire, which erupted nearby in early October, damaged or destroyed 80 structures, including homes, in the inland suburbs southeast of Los Angeles.

The original fire is believed to have started accidentally in a roadside shrub while a California Department of Transportation crew was cleaning a section of the State Route 91 freeway with a street sweeper.

A Caltrans truck trailing the slow-moving sweeper was dropping flares onto the road to warn approaching motorists.

“Based on witness statements and evidence, it is likely that another vehicle struck the flare, causing it to spin off the freeway into the shrub,” Anaheim Fire & Rescue Chief Randy Bruegman told a news conference.

That fire was declared fully contained on Sept. 30. The second fire erupted on Oct. 9 as Santa Ana winds blew through the region at speeds of about 50 mph (80 kph) or greater.

The investigation determined there was a smoldering clump of oak 20 feet within the original burn area, and the strong wind carried embers at least 60 feet (18 meters) into an unburned area and ignited vegetation, Bruegman said.

The wind-driven flames spread through canyon lands among neighborhoods, igniting some homes and bypassing others. Thousands of people fled their homes.

The Orange County Board of Supervisors launched an investigation into whether fire officials did not respond to the second blaze aggressively enough. Bruegman said protocols are under review.

Addressing another controversy about the second fire, the chief said a spot fire reported by a police helicopter crew the previous night on a peak in the Cleveland National Forest was definitively not the cause.

That report ultimately was handled by the U.S. Forest Service, which determined the spot fire was in a remote area, posed no threat to neighborhoods or structures and no action was necessary, Bruegman said.

An ember from that peak would have had to travel several miles against the wind to be the origin of the Oct. 9 fire, he said.

A Caltrans official said the agency worked closely with Anaheim fire authorities during the investigation and has begun reviewing its policies and procedures “to look for ways we can make improvements.”

“We regret that our road flare may have been involved in starting Canyon Fire #1 and recognize the impact this fire had on the community,” Ryan Chamberlain, director of Caltrans District 12, said in a statement.

Canyon Fire 2 broke out as firestorms were ravaging wine country and other areas of Northern California.

“We only need to look at Northern California to see how devastating fire can be, to people, to homes, to communities and to the local economy,” Bruegman said. “And we all know in this business there is no perfect response to any fire situation.”