A 57-year-old man has been charged with arson in one of at least 10 wildfires that erupted in California this week.
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.
Alberto Serrato pleaded not guilty to arson in connection with one of the smaller fires - a 42-hectare fire in suburban Oceanside that started on Wednesday and is fully contained. Bail was set at $US250,000.
Tanya Sierra, a spokeswoman for the San Diego County district attorney's office, said witnesses saw Serrato adding dead brush onto smouldering 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.
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"Unfortunately we don't have the guy that we really want," he said.
Altogether, the wildfires have raced through an estimated 8,000 hectares about 48 kilometres north of San Diego, causing more than $US20 million ($A21.64 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.
On Friday, though, cooler weather aided the 2,600 firefighters, and thousands of people began returning home.
