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Reinforcements from other regions are helping firefighters contain more of the largest wildfires devastating Northern California, though strong winds expected over the weekend could challenge those gains, a fire chief said Friday.

Meanwhile, officials are making grim discoveries -- victims burnt beyond recognition -- as they search blackened ruins of some of the 5,700 homes and business that have been destroyed.

"Some of (the remains) are merely ashes and bones," Sonoma County Sheriff Rob Giordano said at a Thursday evening news conference. "And we may never get truly confirmative identification on ashes. When you're cremated, you can't get an ID."

Thirty-three people have been killed since the wildfires began Sunday night, making this outbreak one of the deadliest in state history, according to the California Department of Forestry and Fire Protection (Cal Fire).

Though progress has been made on the big fires, much more work is ahead.

The 48,000-plus acre Atlas fire in Napa and Solano counties was 45% contained Friday -- up from 3% the day earlier. The 44,000-acre Nuns fire in Sonoma County -- an amalgamation of three recently merged fires north and west of Glen Ellen -- was 5% contained.

The 34,000-acre Tubbs fire in Napa and Sonoma counties was 44% under control. The 34,000-acre Redwood and Potter fires in Mendocino County were 10% contained.

Firefighters are making progress on some of the bigger fires, Napa County Fire Chief Barry Biermann said Friday, thanks in large part to the gumption of those who've been on the lines for days and the reinforcements who are relieving them.

"It's like pulling teeth to get law enforcement and firefighters to disengage from what they're doing out there -- they're truly passionate about what they're doing to help the public. But the reinforcements are coming in, and that's why you're seeing the progress that we're making," Biermann said.

Since Sunday, the deadly fires have consumed thousands of homes and forced evacuations in Northern California's wine country and produced unhealthy air quality in the San Francisco Bay Area.

Latest developments

• Of the 36 people who were killed by the fires since Sunday night, 19 died in Sonoma County, officials said. Nine people in Mendocino County, at least four in Yuba County and four in Napa County have died, officials said.

• More than 2,800 residences in Santa Rosa have been destroyed by wildfires, Mayor Chris Coursey said. The number of destroyed structures in the state went up Friday by 2,200 to 5,700, Cal Fire said.

• Wildfires have burned more than 221,000 acres throughout California; 17 wildfires remained Friday, Cal Fire said.

• Winds could be especially gusty Friday night through Saturday. Those conditions, joined with low humidity, could spread the flames drastically, the National Weather Service warns. "Very dangerous #fire conditions expected overnight (Friday) due to strong winds and ongoing fires. Please be prepared & aware!" the weather service tweeted.

• The causes of the fires are under investigation. But officials said their spread was aided by strong winds Sunday night, with some gusts of more than 70 mph.

• About 34,000 utility customers are without electricity service -- and natural gas service to 47,000 customers has been shut off -- mostly in Sonoma and Napa counties, the Pacific Gas and Electric Co. said Friday.

Many of the fire victims are elderly

A total of 235 people are reported missing in Sonoma County alone, where a fire wiped out many homes in Santa Rosa, a city of about 175,000 people some 50 miles northwest of San Francisco. t

Deputies are having to wait for houses to cool before they can enter to look for the missing, said Giordano, the sheriff.

The Sonoma County Sheriff's Office identified 10 victims Thursday, and most of them were over 70. The youngest was 57, and the oldest was 95.

Sonoma, Mendocino and Napa counties have been among the hardest hit by the fires. Nearly 20% of the population in those counties is over 65, according to US Census Bureau data.

In Sonoma, authorities had to turn to dental records, fingerprints, tattoos and serial numbers on hip implants to identify victims.

"We've been forced to work that direction because we may not have enough information to identify people because of the ... severity of the burn," Giordano said.

On Friday, cadaver dogs and searchers went through what was left of a mobile home park in Santa Rosa.

"We start with a bedroom because this fire occurred at night we think a lot of people were in their bedrooms," said Sonoma County Sheriff's spokesman Spencer Crum told CNN affiliate KOVR of Sacramento.

There were about 300 mobile homes in the neighborhood.

Pregnant woman flees on bike

Evacuees who escaped oncoming flames described harrowing ordeals.

Charity Ruiz, who is pregnant, had been in a car with her family trying to evacuate from their Santa Rosa neighborhood but got stuck in a traffic jam.

"Honestly, I've never in my life felt like I was going to die like that moment," Ruiz told CNN affiliate KPIX. "Not just me, but my girls and unborn baby."

Unable to wait any longer, Ruiz walked back and got her bike from her house and rode out of the neighborhood with her two girls in the toddler trailer.

"I can ride a bike, but I'm pregnant so it was hard," she told the station. Ruiz had been scheduled to have her baby next week.

She and her kids made it out of harm's way, but their home burned to the ground.

'Peanuts' creator's home lost to wildfire

The fires have been fast and ruthless, shifting without much notice and destroying thousands of structures.

One of them is the Santa Rosa home of Charles Schulz, the creator of the comic strip "Peanuts." Schulz died in 2000.

His widow, Jean, 78, evacuated from the home Monday, shortly before the flames reduced it to rubble, his son Monte Schulz said. The fire destroyed precious reminders of the life his stepmother and his late father had built together, along with memorabilia, Schulz told CNN.

Victims ponder next moves

Many who lost homes are trying to figure out what to do next. In Yuba County, where a 10,000-acre blaze has killed at least four people, Mariano and Christa Domingo saw the fire approach their fence Sunday night, and they drove away with only one emergency oxygen tank for Christa, who has lung problems, CNN affiliate KOVR reported.

"She was thinking we had to pack up things, and I said, 'No, we don't have any time,' " her husband told the TV station.

Their house was destroyed, and the couple only have clothes they received from a shelter. They said they intend to rebuild their home.

"I'm lucky," Christa Domingo told KOVR. "And I've still got my family."

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