LOS ANGELES — The "House of Contentment" has vanished, and its elderly owners are missing — just two of the 13 people who still can't be found after muddy rainwater and debris killed at least 17 people in Southern California.

Bill 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, the couple's granddaughter.

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 Rd. had been a sunny place filled with the artworks collected by Alice Mitchell, who steadied art in college, Megan Mitchell told NBC News on Wednesday.

"My dad [the Mitchells' son] texted to say he'd been trying to contact them," Megan Mitchell said. But when a relative went to the sheriff's office for information, "they told her there was no 319 Hot Springs Rd. anymore."

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, their granddaughter said.

She's still holding out hope that her grandparents are alive somewhere. But they're in their late 80s, and they have difficulty moving.

"My grandfather was really funny," Mitchell said. Then she stopped herself. "Is funny," she corrected.

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 13 others who couldn't be accounted for amid what was morphing into a massive cleanup effort.

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.

Interstate 101, California's main north-south coastal highway, were buried under mud and debris, and at least 30 miles of the southbound lane will be closed through Monday at the earliest, the state Transportation Department said.

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 pushed off their foundations or buried up to their rooftops. Car-size 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.

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.

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.

A post shared by Oprah (@oprah) on

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.

When the rains came, "all hell broke loose," Peter Hartmann, a dentist who moonlights as a news photographer for the local website Noozhawk, told The Associated Press.

He said he watched rescuers revive a toddler who was pulled unresponsive from the muck. "It was a freaky moment to see her just covered in mud," he said.

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