A 57-year-old man has been charged with setting one of 10 wildfires that swept the San Diego region this week.

Alberto Serrato pleaded not guilty Friday to arson in connection with a 42-hectare fire in suburban Oceanside — one of the smaller fires. The fire started Wednesday and is fully contained.

hi-cali-fire-852.jpg

A Los Angeles firefighter walks down a hillside burnt out by the Cocos Fire in San Marcos, California, on May 16, 2014. Several thousand suburban San Diego residents forced to flee a wildfire threatening their towns were allowed to return home early on Friday as firefighters battled for a third day against a swarm of blazes burning in and around California's second-largest city. (Mike Blake/Reuters)

Tanya Sierra, a spokeswoman for the San Diego County district attorney's office, says witnesses saw Serrato adding brush to the flames near homes but he was not seen starting the fire. The spokeswoman says he has not been connected to any other fire.

The spokeswoman says Serrato was arrested Thursday in Oceanside and faces up to seven years in prison if convicted.

State fire officials said the first of at least 10 blazes that erupted between Tuesday and Thursday was found to have been caused by a spark from malfunctioning construction equipment. But it could take months to get to the bottom of the most damaging fires.

"We are not ruling out anything," San Diego County Sheriff Bill Gore said.

All together, the wildfires have raced through an estimated 8,100 hectares about 48 kilometres 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, 80 km/h winds and temperatures in the 90s. On Friday, though, cooler weather aided the 2,600 firefighters, and thousands of people began returning home.

Hotline for suspicious activity

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."

'Do people have suspicions? Yes. But can we confirm them? The answer is no.' - Police Capt. Neil Gallucci

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 121,400 hectares 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 cancelled graduation ceremonies, and Carlsbad, where the Legoland amusement park was forced to close.