Heavy rain on Tuesday, which followed 10 months of drought, sent sticky mud and debris flowing from the hills into Montecito and other towns in Santa Barbara County, northwest of Los Angeles.
"There are 43 people we’re now looking into and investigating to see if they actually are missing," Santa Barbara County Sheriff Bill Brown said, while adding: "It’s a constantly moving number."
Brown explained that those unaccounted for could yet be located, but warned that the death toll could rise.
Authorities emphasised that the situation was still fluid. Earlier in the day, officials put the number of missing at just eight.
"There are mothers, fathers, grandfathers, siblings, and the list goes on and on," Brown said.
Terrifying wildfires forced people to evacuate in December -- with the mudslides striking just two weeks after they returned.
The fires burned most vegetation, leaving perfect conditions for the latest tragedy to unfold.
With clean-up operations underway, workers were battling the viscous mud on the streets of Montecito Thursday -- while in other areas, efforts continued to restore power.
"It was a scary night," local resident Jane Barret said. One of her doctors, the leader of her children’s scout group and a former neighbour were among the dead.
Barret added it would "take a lot of time" to get over the natural disasters that have struck the area over the past two months.
Heavy rain, destroyed homes
The highest rainfall was recorded at five inches (13cm) in Ventura County, according to the National Weather Service -- but forecasts for the next week do not predict rain.
Residents were rescued by helicopter from the roofs of their homes -- with some later returning to recover valuable belongings and pets.
Shocked by the damage, they took photos of their now-destroyed houses, with many describing the storm as similar to a stampede or an out-of-control train.
Greg Duimovich said his house had "never" shaken so much from a rainstorm.
More than 50km (35 miles) of the 101 Freeway, which connects northern California to the south, remained blocked by mud.
Billionaire talk show queen Oprah Winfrey -- whose rousing speech at Sunday’s Golden Globes sparked speculation about a possible White House run -- was among those affected.
She took to social media to post a video which showed mud rising up to her knees at her Montecito mansion, where fellow talk show host Ellen DeGeneres and actor Jeff Bridges also own homes.
"Our home has been severely damaged, but we are safe," Bridges said on Twitter. VNS