At an afternoon press conference, county Sheriff Bill Brown said authorities were investigating whether 43 people were missing. He cautioned that it was unclear how many of those persons were "actively missing."
Santa Barbara County spokeswoman Amber Anderson said the number of confirmed fatalities remains at 17. Overall, 28 people were injured. Twelve remained hospitalized, four in critical condition.
The Santa Barbara County coroner's office on Thursday released the names of the deceased. All are residents of Montecito and range in age from 3 to 89.
More than 1,560 personnel were assigned to the search-and-rescue and cleanup efforts in an incident area that spanned 30 square miles. They were supported by K-9 units, 10 helicopters and six bulldozers.
VIDEO: River of mud rushes down Montecito street as family scrambles for safety
Army Corps of Engineers personnel are expected to arrive in the area late Wednesday, along heavy machinery to remove boulders and clear out storm drains.
Three people were rescued on Wednesday. Searchers believe they have looked through about 75 percent of the debris field.
Rescue crews worked up to 12 hours a day and risked stepping on nails or shattered glass, or being exposed to raw sewage, or dealing with leaking gas, said Deputy Dan Page, chief of a Los Angeles County Sheriff's Department rescue team.
"We've gotten multiple reports of rescuers falling through manholes that were covered with mud, swimming pools that were covered up with mud," said Anthony Buzzerio, a Los Angeles County fire battalion chief. "The mud is acting like a candy shell on ice cream. It's crusty on top but soft underneath, so we're having to be very careful."
After a better look at the damage, officials lowered the number of destroyed homes from 100 to 64 and raised the number of damaged ones from 300 to 446.
Santa Barbara County authorities sent a shudder through the community early Thursday morning when they reported that the number of people unaccounted for had surged to 48. But later in the morning, they said they had made a clerical error, and the actual number of missing was down to eight.
As the day wore on, the search for the missing became an increasingly desperate exercise, with growing doubts about whether anyone would be found alive.
"They're not going to find survivors anymore. They're going to find bodies in the mud," said area resident David Weinert, who feared two of his neighbors were among the dead and turned out to be right in at least one case. "It's emotional for me to say this, but I think they're gone."
Flooding and debris flow prompted the California Highway Patrol to shut down the northbound 101 Freeway at Route 126 in Ventura and at Seacliff, and the southbound 101 Freeway just south of Santa Barbara at Milpas.
The closure was expected to remain closed at least through midday Monday, the CHP said.
The Santa Barbara County Public Health Department on Thursday afternoon ordered the closure of more than a dozen beaches, stretching from Gaviota south to the county line.
The agency cited high bacteria levels and hazardous materials on the beaches as reasons for the closures.
A new mandatory evacuation order went into effect at 6 p.m. The affected area was located west of Sheffield Drive, East Valley Road and Lader Lane; east of Olive Mill and Hot Springs Road, north of the ocean and south of U.S. Forest Service boundary.
Residents will not be allowed to return to their homes while the evacuation order is lifted, which may not happen for one or two weeks, officials said.
At the Montecito Inn, a storied hotel built by Charlie Chaplin, Andrew Becker and employees of Becker Construction were working Thursday evening to remove about eight feet of mud from the basement.
"They found someone here early this morning, before we got here," Becker said. "The fire department keeps coming in and checking on us. We're supposed to call them if we find any bodies."
Cleaning out the first floor alone required 100 trips of trucks filled with mud and debris.
"It's a thousand yards of dirt," Becker said. "It's pretty unbelievable."
For days, the county had issued repeated warnings via social media, news media and emails about the potential for mudslides. But county emergency manager Jeff Gater said officials decided not to use the cellphone push alert system until 3:50 a.m. Tuesday out of concern it might not be taken seriously.
The first slides ripped through Montecito about 3:30 a.m. and continued after the county cellphone alerts went out.
"It was activated at the appropriate time actually when the event was occurring," Brown said.
Only an estimated 10 to 15 percent of residents fled when ordered, and much of the damage occurred where evacuations were voluntary.
Kelly Weimer, whose parents were killed when their Montecito home was swept away by flash floods, said they "were in a voluntary evacuation area so they figured they were OK."
"They weren't concerned," Weimer said. "It's not like anybody came around and told them to leave."
The Associated Press contributed to this report.