SAN MARCOS -- Firefighters aided by calmer winds made progress Thursday against a series of wildfires burning across San Diego County, and authorities collected clues and solicited the public's help to determine what caused so many blazes to occur simultaneously.
While some of the nine fires were extinguished and thousands of people were able to return to their homes, the San Marcos blaze roared back in the afternoon.
Flames raced along scrubby hillsides as massive black plumes filled the skies.
Smoke limited visibility to a few feet at times in the city of 85,000 about 35 miles north of San Diego. On one street, five horses wandered nervously in a paddock as firefighters worked to protect nearby homes and barns.
Sheriff Bill Gore said the flare-up prompted more than 13,000 new evacuation notices and served as a "reminder to everybody just how volatile this can be."
The fires have destroyed at least eight houses, an 18-unit condominium complex and two businesses, and has burned more than 15 square miles, causing more than $20 million in damage so far.
Most of the damage was in San Marcos and Carlsbad. No major injuries were reported.
Bay Area crews have headed south to help fight the blazes. A team consisting of firefighters from Fremont, Oakland, Hayward, Livermore-Pleasanton and Alameda County Fire deployed to San Diego County on Thursday, Hayward Fire spokesman Don Nichelson said.
While drought conditions and unusually high temperatures made the area ripe for wildfires, there are suspicions that at least some of the blazes might have been set. Gore said arson is being looked at but so are many other possibilities, such as sparks from vehicles.
He encouraged the public to contact authorities with any information.
Since the fires began Tuesday, 125,000 evacuation notices have been sent. Schools across the county were shut down, and the Legoland amusement park had to close Wednesday. It reopened Thursday.
While local authorities congratulated themselves for the cooperative effort among agencies and the bravery shown by firefighters, not everyone was pleased.
Greg Saska stood in front of his charred Carlsbad home Thursday in sandals that showed his soot-covered feet. He said he was not impressed with the fire response.
"I don't want to complain, but I wish they had just made a little more effort to put the fire out," Saska said. "The end of the house ... was still burning. And they (firefighters) just left. And I'm just kinda going, 'What would've been the big deal to stay here another 10 minutes and put that out totally?' I just don't get it."
The flare-up in San Marcos ran up a slope in a heavily vegetated area but with no wind on it.
The fire was being driven by fuel and topography, said Division Chief Dave Allen of the California Department of Forestry and Fire Protection.
"It's created its own weather pattern there as it sucks oxygen in," he said.
State fire Capt. Kendal Bortisser said the fire was running east along hillsides behind California State University, San Marcos, which canceled graduation ceremonies because of the danger from the flames.
The blaze was only 5 percent contained by late Thursday afternoon.
Firefighters have been contending with sweltering heat in their fight to contain flames fueled by brush and trees left brittle by drought. Temperatures on Thursday ranged from the high 90s to 100.
Calmer winds allowed aircraft to make a heavy contribution to the firefighting efforts.