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LOS ANGELES — The House of Contentment has vanished, and its elderly owners were confirmed Wednesday to be among the 17 people killed as muddy rainwater and debris swept through Southern California this week.

Jim and Alice Mitchell moved into their dream retirement home in the ritzy Montecito area of Santa Barbara County in 1999. They'd painted the phrase "Case de Contenta" on the side — "House of Contentment," in Alice Mitchell's words, according to Megan Alice Mitchell, their granddaughter.

Alice and Jim Mitchell, seated at right, in a 1991 picture with members of their family. Courtesy Margaret Mitchell

The Mitchells' family learned Wednesday that they were dead, Megan Mitchell told NBC News. Their bodies were found Tuesday morning on a different street, added Clay Weimer, a son-in-law.

Only last month, the Mitchells had evacuated the house because of the threat of wildfires racing through the region. They'd moved back home only about a week ago, Megan Mitchell said. They chose not to leave this time, she said.

Until it was washed away this week by raging floods, leaving only a moonscape of mud and rocks, the three-bedroom Spanish-style house at 319 Hot Springs Road had been a sunny place filled with the artworks collected by Alice Mitchell, who studied art in college, Mitchell told NBC News.

"My dad [the couple's son] texted to say he'd been trying to contact them," Mitchell said.

But when a relative went to the sheriff's office for information, she said, "they told her there was no 319 Hot Springs Road anymore."

While the worst of the rainstorm early Tuesday that closed roads and cut power to thousands of people was over, Montecito was grappling with the deaths of at least 17 people and trying to find eight others who were officially unaccounted for amid what was morphing into a massive cleanup effort. Authorities in Santa Barbara briefly said there were 48 people reported missing but quickly corrected that figure. They said the mistake was due to a clerical error.

Another victim was identified Thursday as Montecito man Peter Fleurat, according to his niece.

Santa Barbara County Sheriff Bill Brown lamented what he called "another extremely challenging day as we find out about more of our community members who were killed."

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"This is going to be a long and difficult journey for all of us," Brown said at a news conference Wednesday afternoon.

Hayden Gower couldn't hold back his tears as he frantically searched for his mother, Josie Gower, 69.

"The water and the mud just flew in and took her by surprise," he told NBC News on Wednesday. "I don't know what to do. I don't know. They haven't found her. I'm going to do everything I can."

More than 100 homes were destroyed in the flash flooding in Montecito and surrounding communities, according to the county. About 7,000 people remained under mandatory evacuation orders. About 6,000 homes and businesses were without power, and many areas still had no water or sewer service.

Highway 101, California's main north-south coastal route, was buried under mud and debris, and about 30 miles of the southbound lane will be closed at least through midday Monday, said Capt. Cindy Pontes, Santa Barbara County commander for the California Highway Patrol.

The region's hillsides were already scorched from last month's Thomas Fire, the largest wildfire in state history, which made the area more susceptible to flooding because of the lost vegetation.

With 4 to 5 inches of rain falling in a matter of hours, mud poured down hills like a river — so powerful that homes were shoved off their foundations or buried up to their rooftops. Car-sized boulders littered streets.

Debris also ruptured a natural gas line in one neighborhood, causing an unknown number of structure fires. More than 50 rescues took place from the ground, while a helicopter was needed in 50 others, fire officials said, adding that more than two dozen people were also injured.

Related: Luxury Montecito caught off guard by river of mud and debris

Cottage Hospital said it was treating 12 remaining patients, four of whom were in critical condition. Most of the injuries were caused by fast-moving debris, said Dr. Brett Wilson, the hospital's emergency director.

But there were also glimmers of optimism amid the fury of the flood.

A 14-year-old girl was discovered alive when firefighters using rescue dogs heard a scream on her mud-swollen Montecito street. Jaws of life and other tools were used to carefully extricate her.

"I thought I was dead there for a minute," Lauren Cantin, who was shivering and slathered in mud, later told rescuers.

Another survivor, Berkeley Johnson, told NBC affiliate KSBY of Santa Barbara that he and his wife managed to climb to their roof about 3 a.m. (6 a.m. ET) after boulders and mud came crashing through their home. When the flooding receded, they decided to flee.

That's when they heard a baby crying near a neighbor's home, Johnson said.

"We don't know where it came from, but we got it out, got the mud out of its mouth," he told the station. "I hope it's OK."

NBC News couldn't immediately verify his account.

Photos: Rivers of mud sweep through wildfire burn areas in California

Among the residents of Montecito, an unincorporated community northwest of Los Angeles, is Oprah Winfrey. Her home survived, but in a video she posted to Instagram on Wednesday, Winfrey stood in the mud to illustrate the scale of the destruction.

"There used to be a fence right here. That's my neighbor's house. Devastating," she said.

Authorities had warned that destructive floods were likely because areas scorched by California's wildfires last year stripped the region of much of its water-absorbing soil and foliage. But fewer than a fifth of those who were ordered to evacuate did so, authorities said.

Alex Johnson reported from Los Angeles. Erik Ortiz reported from New York.

CORRECTION (Jan. 11, 2018, 12:11 a.m. ET): An earlier version of this article misspelled the last name of Jim and Alice Mitchell's son-in-law. His name is Clay Weimer, not Heimer.