California officials say it will be weeks before they determine the causes of the wildfires sweeping the state.

Ken Pimlott, the state's fire chief, said Friday that 20 investigators are in Sonoma County alone looking into the cause and origin.

He said the process is technical and painstaking, and investigators won't have information to disclose for "weeks to come."

Much of the evidence was consumed in the fires, which means investigators must look for other clues to decide what happened.

They are not yet ready to say whether the fires were caused by downed or sparking power lines from the strong, gusty winds that swept the state overnight Sunday into Monday.

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Thousands of firefighters are battling more than 20 wildfires, with more crews pouring in to help. The fires have killed 36 people. Most of the deaths were in Sonoma County, but fatalities have also occurred in Mendocino, Napa and Yuba counties.

Since igniting Sunday in spots across eight counties, the fires have transformed many neighbourhoods into wastelands, destroying about 5,700 homes and buildings, a significantly higher figure than the 3,500 previously announced.

Sonoma hard hit

Individual fires including the Oakland Hills blaze of 1991 had killed more people than any one of the current fires, but no collection of simultaneous fires in California had ever led to so many deaths, authorities said. The death toll is expected to rise as officials follow up on reports of people who are missing.

Sonoma County Sheriff Robert Giordano said recovery teams would begin conducting "targeted searches" for specific residents at their last known addresses.

APTOPIX California Wildfires

Two firefighters watch for spot fires near Calistoga, Calif., on Friday. Firefighters gained some ground on a blaze burning in the heart of California's wine country but the forecast called for low humidity and high winds, which would complicate efforts. (Jae C. Hong/Associated Press)

"We have found bodies almost completely intact, and we have found bodies that were nothing more than ash and bones," the sheriff said on Thursday. His office had released the names of 10 of the dead, all age 57 or older.

Giordano said about 400 people were on his list of missing people Thursday, although it was unclear how many are duplicates or people who are actually safe.

Some remains have been identified using medical devices uncovered in the scorched heaps that were once homes. Metal implants, such as artificial hips, have ID numbers that helped put names to victims, he said. Distinctive tattoos have helped identify others.

Confusion has marked the disastrous wildfires spanning several counties and cities, adding to the frustration of hundreds of people searching for loved ones. The public is relying partly on separate media updates throughout the day that are broken out by county and agency. Napa County, for example, is advising people to search through a website hosted by the American Red Cross.

'I'm at a loss'

Ellen and Bob Pearson haven't been seen since they were fleeing their mobile home in California wine country, preparing to leave in their purple Pontiac as flames lurked in the distance.

Five days later, no one has heard from the couple, both in their 70s.

"It's been challenging trying to figure out which agency or which number to call," said the couple's granddaughter, Tiffany Couto.

"People are trying to help so much, but it's a chaotic time, and so it's a challenge to understand exactly how to handle this."

California wildfires

The deadliest blazes have occurred near Santa Rosa in Sonoma County, but deaths have also been reported in three other California counties.

Couto said her family reported her grandparents' distinctive purple Pontiac, but she doesn't know whether officers ran the licence plate numbers in their search for missing people.

"I'm at a loss, and I'm not sure what steps to take to find them," she said. "We're all confused. We're not sure how to be productive."

Rick and Leslie Howell were among those found unharmed. Family located the couple Wednesday after they fled their Santa Rosa home in a hurry a day earlier. They do not own mobile phones, which made it difficult to let friends and family know they were safe.

Events cancelled due to air quality

Those not trying to find lost loved ones are sifting through the remains of lost homes.

"It wears you out," said winemaker Kristin Belair, who was driving back from Lake Tahoe to her home in Napa. "Anybody who's been in a natural disaster can tell you that it goes on and on. I think you just kind of do hour by hour almost."

"We had series of statewide fires in 2003, 2007, 2008 that didn't have anything close to this death count," said Daniel Berlant, a deputy director with the California Department of Forestry and Fire Protection.

Smoke from the wildfires has caused air quality levels to plunge in the Bay Area around San Francisco, sending people to emergency rooms and forcing schools to close and people to wear masks when they step outside.

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"We have unprecedented levels of smoke and particles in the air," said Ralph Borrmann, a spokesperson for the district.

He called it the worst air quality ever recorded in many parts of the Bay Area.

At an Ace Hardware store in San Francisco's financial district, phones were ringing nonstop with customers looking to buy breathing masks. They were sold out, as were most stores in the area.

With winds expected to keep blowing in smoke from the fires to populated areas this weekend, many schools decided to close Friday and organizers cancelled weekend events, including an Oktoberfest in Walnut Creek and a fitness festival and half marathon in San Francisco.

The NFL has been exploring options to move Sunday's game in Oakland between the Raiders and Los Angeles Chargers if necessary. Officials at the University of California, Berkeley and Stanford are monitoring the air quality as weekend college games approach.

California Wildfires The Missing

Chrystal Couto holds a poster of her grandmother as Aaron Austin looks on in Santa Rosa, Calif., on Friday. The couple from nearby Redway are searching for Couto's grandmother and her husband, who haven't been heard from since a wildfire tore through the older couple's Santa Rosa neighbourhood. (Paul Elias/Associated Press)