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As teams frantically searched areas of Southern California smothered by rivers of mud and boulders, officials on Thursday increased the size of the evacuation zones.

The immediate areas where at least 17 people were killed by Monday’s mudslides are under mandatory evacuation as of 6 p.m., Santa Barbara Sheriff Bill Brown said.

“We know that this a terribly inconvenient development but it is also incredibly necessary,” Brown said to residents.

The number of people missing in and around the oceanside community of Montecito was in flux Thursday. At an afternoon news conference Brown said as many as 43 people could be considered missing.

Authorities have not confirmed that many people are unaccounted for, he said, but are looking into reports from phone calls, social media and message boards.

“It doesn’t mean that they are all actively missing,” he said.

Thursday morning the total was reported as 48 then revised to eight due to a “clerical error,” according to the sheriff’s office.

Latest developments

• Victims list: Officials have released the names of the 17 people killed. The victims ranged from 3 years old to 89 years old.

• New Evacuation Order: A new mandatory evacuation order is in place in Montecito for the area west of Sheffield Drive/East Valley Road/Ladera Lane, east of Olive Mill Road/Hot Springs Road, north of the ocean and south of the U.S. Forest Service boundary. Anyone in the area who needs assistance evacuating is urged to call 833-688-5551. Residents will need be allowed to return home once they evacuate, and the order may be in place for one to two weeks. (A map of the evacuation zone can be found here

• Deadly storm: All the deaths were reported in Santa Barbara County, authorities said. An additional 28 people were injured in the county.

• Destruction: At least 65 homes and 462 other residences were damages in Santa Barbara County, spokeswoman Susan Klein-Rothschild said Thursday. Eight commercial buildings were destroyed, and 20 were damaged.

• Search: Crews had completed a primary search of 75% of the debris field as of Wednesday, and more than 500 first responders and 10 dogs are looking for victims in Santa Barbara County.

• Water: A boil water notice is in place for Montecito and Summerland.

• Weather conditions: No rain is forecast for the area over the next seven days. High temperatures are expected to be in the upper 60s to mid 70s from Thursday through the weekend.

• Road closed: Sections of US 101, a major thoroughfare connecting Northern and Southern California, are closed until Monday.

Race against time

Anxious residents have been awaiting word on their loved ones. On Wednesday, Brown said the priority is finding survivors.

Rescue workers are using helicopters in a search hampered by blocked roads and downed trees and power lines.

In Montecito, rescuers have been tearing through mounds of mud, furniture and fallen trees to search for those trapped.

“We still have to have hope and believe that people can be found. There (are) dogs everywhere searching, but I truly believe we are going to find more people alive,” Montecito resident Curt Pickering told KSBY on Wednesday.

Some received good news of disoriented loved ones rescued from the roof of their muddied, flooded home. But others were not so lucky.

Diane Brewer said her friend, Josie Gower, 69, died after she opened her door and was swept away by the mudslide.

“It was always a full life with Josie. Now, it’s just a hole,” she said.

Catholic school founder Roy Rohter, 84, and his wife, Theresa, were swept from their Montecito home. Rohter died, but his wife was rescued, said Michael Van Hecke, headmaster of St. Augustine Academy in Ventura, which Rohter founded in 1994.

Rebecca Riskin, the founding partner of Montecito real estate company Riskin Partners, also died, the company said. Riskin began selling real estate in Los Angeles and moved to Montecito nearly three decades ago.

Her loss is “incredibly devastating to her friends, family and our community,” the company said Wednesday.

Authorities have not released the names of the missing, but some of their relatives have confirmed their identities.

James and Alice Mitchell, an elderly couple from Montecito, are among those missing, their granddaughter, Sarah Weimer, said Wednesday.

Workers hike to keep senior home running

Besides damaging homes, the mudslides interrupted power and water service for parts of the Montecito area.

And with blockages making driving difficult, some are hiking or biking to work — including staff at a Montecito retirement home, CNN affiliate KEYT reported.

“We do have key staff there. … We just want to let everyone know that all residents are accounted for and safe and sound,” Katie Hoegh of the Casa Dorinda retirement home told KEYT on Wednesday night.

With parts of US 101 closed, motorists moving up or down the coast must choose between time-consuming alternate routes. Some will drive inland to access Interstate 5.

Others are taking boats between Ventura and Santa Barbara to get around the closure.

Some companies were offering ferries about four times a day, carrying roughly 400 people daily.

Nannette Clark of Goleta said she used the ferry to visit her grandfather for his 100th birthday in Ventura. Driving around the closure would have taken hours longer, she said.

“The only way to get down here today (in a reasonable time) was to take” the ferry, Clark told KTLA.

Amtrak service in Santa Barbara is suspended through at least Thursday as crews work to reopen train tracks.

Burbank, just north of Los Angeles, also had problems with mudslides.

One resident of the town, whose escape in a Prius went viral after firefighters posted a video of it, told KTLA he didn’t think he was going to make it.

Desionne Franklin told the station that his housemate told him about a mandatory evacuation. So he and he girlfriend dug out the driveway and he eased down the road at first.

“When I got halfway down the hill, I just felt this burst of water hit the back of the car,” he told KTLA.

The Toyota hydroplaned the rest of the way down the hill, he said.

“I guess this is going to be it for me, right here,” he said he thought, but the car got traction just in time to avoid a crash.

Geologist: Risk of mudflow remains for years

The rain poured down on hillsides charred by recent wildfires, which burned vegetation that otherwise could make the terrain more resistant to mudslides.

The Thomas Fire — the largest wildfire in California’s recorded history — has burned more than 281,000 acres in Santa Barbara and Ventura counties since it began in early December. It’s still burning.

Geologists and forecasters warned that intense rain could trigger deadly mudslides from the scorched areas.

And because of the fire, communities below the scarred terrain could remain at risk of mudlfow for a few years, said Randall Jibson, a research geologist with the US Geological Survey.

Montecito may be at slightly less risk now, because this week’s flooding already brought down vulnerable material.

“(But) no storm brings down everything that is susceptible. There’s almost always more” that could come down, he said.

What can be done? Long term, one option would be more basins to slow down storm runoff and collect debris.

Short term, making the public ready to evacuate during heavy rains is key, he said.

Montecito and Carpinteria are especially vulnerable to mudslides because the steep terrain in some places goes from thousands of feet above sea level to sea level in “a matter of just a few miles,” said Tom Fayram, a deputy public works director with Santa Barbara County.

The rain fell at more than 1.5 inches per hour at one point early Tuesday in parts of Southern California. About a half inch per hour is enough to start mudslides, said Robbie Monroe of the National Weather Service in Oxnard.

Mudslides are not uncommon to the area. In January 2005, a landslide struck La Conchita in Ventura County, killing 10 people.

Pounding rain

The rain poured down on hillsides stripped of their vegetation by the massive blaze that started last month.

The Thomas Fire — the largest wildfire in California’s recorded history — has burned more than 281,000 acres in Santa Barbara and Ventura counties since it began in early December. It is 92% contained, and officials don’t expect full containment until later this month.

Without vegetation to make the terrain more resistant to mudslides, boulders and other debris rolled down onto roads and homes Tuesday.

‘Many chose to stay in place’

The storm hit hard between 3 and 6 a.m. Tuesday. Before the storm hit, Santa Barbara issued mandatory evacuations for 7,000 people, and thousands more homes were in voluntary evacuation zones, including in parts of Carpinteria, Montecito and Goleta, which are below areas scorched by wildfires, county spokeswoman Gina DePinto said.

“While some residents cooperated with the evacuations, many did not. Many chose to stay in place,” said Brown, the sheriff.

But the area where most homes were destroyed, south of Highway 192, was not in a mandatory evacuation zone.