SAN DIEGO » A 57-year-old man was charged with arson Friday in one of at least 10 wildfires that erupted in Southern California this week, and investigators were working to determine whether other blazes in the unusually early and intense outbreak were ignited by something as ordinary as sparks from cars or something more sinister.

State fire officials said the first blaze that erupted between Tuesday and Thursday was caused by a spark from malfunctioning construction equipment. But it could take months to get to the bottom of the most damaging fires.

The man charged Friday, Alberto Serrato, pleaded not guilty to arson in connection with one of the smaller fires — a 105-acre fire in suburban Oceanside that started Wednesday and is fully contained. Bail was set at $250,000.

Tanya Sierra, a spokeswoman for the San Diego County district attorney’s office, said witnesses saw Serrato adding dead brush onto smoldering bushes, which flamed up. He has not been connected to any other fire, Sierra said.

Oceanside police Lt. Sean Marshand said Serrato is believed to have added fuel to the fire but not to have started it.

"Unfortunately we don’t have the guy that we really want," he said.

Serrato was booked into jail Wednesday, according to the San Diego County Sheriff’s Department website, but his arrest wasn’t announced until Friday. Sierra didn’t know if he had an attorney.

All together, the wildfires have raced through an estimated 20,000 acres about 30 miles north of San Diego, causing more than $20 million in damage. One burned body was found in an encampment of homeless people. At least eight houses and an 18-unit condominium complex were destroyed, and tens of thousands of people were warned to leave their homes.

Eight of the blazes popped up between late morning and sundown on Wednesday, raising suspicions that some had been set.

The region has become a tinder box in recent days because of conditions not normally seen until late summer — extremely dry weather, 50 mph Santa Ana winds and temperatures in the 90s. On Friday, though, cooler weather aided the 2,600 firefighters, and thousands of people began returning home.

In one of the hardest-hit cities, Carlsbad, investigators finished examining the burn site across the street from a park and focused on interviewing people who called a hotline that was set up to report any suspicious activity.

"Do people have suspicions? Yes," said police Capt. Neil Gallucci, noting there has been no lightning that could explain the blazes. "But can we confirm them? The answer is no."

Police in the city of Escondido arrested two people, ages 17 and 19, for investigation of arson in connection with two small fires that were extinguished within minutes. But they found no evidence linking the suspects to the 10 bigger wildfires.

The list of possible causes is long.

"Our investigation might be over quickly for some of these fires — say, if we find a piece of metal nearby from a catalytic converter that backfired," the sheriff said. "But others might not be so easy to determine. We’ll be talking to people in the areas to see if they saw anything to see if arson might have had a role."

Investigators will visit each burn site and go down a list, marking what they know and don’t know.

Is it near a road? That raises the possibility that the flames were ignited by a hot tailpipe, sparks from a catalytic converter or a discarded cigarette from a motorist. Is there a railroad nearby? Are there any power lines?

Investigators will also study the ground for footprints or tire tracks and analyze the burn pattern.

Two of the blazes broke out at military bases, where training exercises with gunfire have been known to spark flames.

A 2003 wildfire in Southern California that killed 15 people, destroyed more than 2,000 homes and blackened 300,000 acres in October and November was caused by a lost hunter who set a signal fire. Sparks from power lines were blamed for wildfires in the San Diego area in 2007 that left five people dead and burned down about 1,500 homes.

This time, the hardest-hit areas were in the cities of San Marcos, where a college campus shut down and canceled graduation ceremonies, and Carlsbad, where the Legoland amusement park was forced to close.

A dozen wildfires popping up in a single day is not unheard of in the drought-stricken state, but it’s a phenomenon usually seen during the dog days of summer.

"What makes the San Diego area fires so unique is that we had tinder-dry conditions and Santa Ana winds in the month of May, and that’s unprecedented," state fire spokesman Daniel Berlant said.

Spagat reported from Escondido, California.