PARADISE, Calif. — Wendy Bailey, a widow and retired stay-at-home mother, hunched over a charred bathtub, surrounded by ash, looking for any trace of human remains.

It was her second day searching for victims of the ferocious wildfire that destroyed this city of about 26,000 residents last week, an effort that authorities say has become the largest search operation in California history.

As Bailey shifted her fingers through the rubble, in a subdivision where every house was leveled by flames, the scale of the task facing her and hundreds of other search-and-rescue volunteers began to sink in.

That charred piece of glass. Is it a tooth? The fingernail size piece of stucco. Is it a bone?

“We have never had anything of this magnitude,” said Bailey, 58, whose team had found the remains of two victims the previous day. “I have seen burned bodies before, but never just disintegrated. It’s usually not like this.”

A week after the Camp Fire was sparked in Northern California, the death toll from the state’s deadliest wildfire in its history continues to grow. At least 63 people have died in the fire, officials said, including the remains of seven people discovered Thursday. But search teams continue to sift through an estimated 10,000 destroyed structures for signs of the people who remain unaccounted for, an ever-changing list of names amid the frenzy of new and canceled missing-persons reports. The number of missing people increased dramatically, to 631.

“You have to understand, this is a dynamic list,” Butte County Sheriff Kory L. Honea said. “Some days might be less people, some days might be more people, but my hope at the end of the day, we have accounted for everybody.”

Some remains may never be recovered, the sheriff said.

With this hilltop community still smoldering, California authorities are leaning on volunteers such as Bailey for what is being called the largest body-recovery mission in state history, and one of the largest in the United States since the 9/11 terrorist attacks.

After Honea put on a statewide appeal for help earlier this week, more than 450 members of California search-and-rescue teams have come here offering to help. They represent nearly all of California’s 58 counties, highlighting the effectiveness of a state law that mandates each county sheriff maintain volunteer search-and-rescue teams.

The volunteers are a variety of ages, both male and female, “a cross-section of our community,” as Ben Ho, who coordinates ­cadaver-dog teams for the California Governor’s Office of Emergency Services, put it.

Bailey joined the effort eight years ago, as a volunteer in an aquatic search-and-rescue unit in Southern California. Her unit, based in Kern County, usually responds to water rescues in the Kern River or missing hikers in the lower Sierra Nevada mountains, but has been called in to other areas with large-scale disasters.

Even here in California, where the concept of search and rescue dates back to efforts to rescue settlers who went missing while crossing the Sierra Nevada mountains in the 1800s, the scale of the Camp Fire response is testing first responders. As the number of state fire disasters continues to grow, state emergency officials say communities here and nationwide need to step up drills and training for how to effectively use volunteer search-and-rescue teams in natural disasters and other mass-causality events.

Many cadaver dogs are not as prepared as they should be to work safely in ash, officials said. California emergency managers also continue to refine how volunteer search and rescue should be deployed, and under what command.

“Each one seems to be much more intense, and also the expectation of the public, the families, the agencies, is that we can do a number of tasks that are very challenging,” Ho said. “This is daunting — both physically and mentally.”

Ho said the modern-day search-and-rescue team can be traced back to the 1989 Loma Prieta earthquake, which devastated parts of San Francisco Bay area. At the time, Ho was a search manager for the Oakland, Calif., fire department. After both San Francisco Bay area communities and federal officials struggled to respond to the earthquake, President George H.W. Bush pressed Ho and other regional emergency managers to develop a more effective plan for search and rescue, including more cadaver dog teams.

“He said we need to have the SWAT teams of rescue teams throughout the country and go to big disasters,” Ho said.

After a few years of planning, including the formation of committees Ho participated in, ­modern-day urban search-and-rescue teams were formed. States and localities also greatly expanded use of cadaver dog units.

Those teams, which often operate in conjunction with the Federal Emergency Management Agency, have been front-line responders to disasters, such as 9/11 and Hurricane Katrina in 2005.

There has also been a proliferation of volunteer wilderness search-and-rescue teams, especially here in California. Christopher Boyer, executive director of the National Association for Search and Rescue, said wilderness teams are better suited to respond to the Camp Fire due to the scope of the disaster, which has charred about 140,000  acres — nearly the size of Chicago.

“The FEMA teams are designed around finding live people, and their dogs find live people,” said Boyer, whose organization has about 16,000 members. In Paradise, “we are talking about finding human remains, and in some cases, cremains that have been burnt.”

Many of the responders to the Paradise fire say they were stunned by the devastation they now must work in.

David Freeman, a search volunteer from El Dorado County near Sacramento, compared his task to working on the moon.

“It doesn’t even seem real,” said Freeman, 75. “We are basically looking for anything that looks like it could be a body, but the fire was so hot, there may not be a lot left there.”

Each morning, the volunteers are broken up into teams of eight to 10 people. If they find suspected human remains, the volunteers have access to anthropologists who help differentiate human remains from animals. Coroner teams are then responsible for removing the bodies.

On Wednesday, Bailey’s team discovered half of a human skull and several bone fragments, roughly the size of a knuckle, she said.

But Honea, who is also the Butte County coroner, has been warning this grief-stricken community that some victims may not be found. The fire was so hot, he said, “it’s possible that some remains were completely consumed by fire.”

“We will continue our search, but at some point, ultimately, with the passage of time and circumstantial evidence will lead us to the conclusion they’ve perished,” Honea said.

The scale of the disaster is even posing problems for cadaver dogs. Although the dogs are trained to sniff out human remains, even those that have been badly burned, public safety officials say the dogs are encountering challenges working in rough terrain that many here fear could be toxic.

“We can’t put booties on their paws because that is like a rock climber with gloves on,” Ho said. “You can’t put masks on them, because they need their noses.”

Mike Delannoy, a volunteer dog handler from Riverside County, spent Thursday morning running his six-year-old border collie, Journey, through a destroyed house. A woman who lives there had been reported missing, but Journey gave only fleeting signals that any remains were located the property, and ultimately none were found.

“The takeaway for us will be we need to focus more on training for large areas with cadaver dog teams,” Delannoy said. “This is an environment where I want to minimize the time he is active in an area.”

But Yerania Molina, a search-and-rescue volunteer from Kern County, said she’s not sure any amount of training could have prepared first responders for what they are encountering in Paradise.

Molina, 37, has a full-time job as an information technology specialist. Since joining Kern County’s rescue team three years ago, Molina has helped recover nine bodies, mostly from the Kern River.

On Thursday, as she prepared for her daily assignment, Molina said she and her fellow teammates approach their jobs with a mix of adrenaline and anxiety.

“You hope you don’t find anybody today because you don’t want any more deaths,” Molina said. “But you also know you that you have to find them because you know they are out there.”