PARADISE, Butte County — Authorities made a distressing revelation Thursday as firefighters gained ground in containing the worst blaze in California history: the number of missing has skyrocketed to 631.

The news, delivered at a news conference Thursday evening by Butte County Sheriff Kory Honea, came after search crews recovered the remains of seven more victims, bringing the death toll in the disaster to 63.

Earlier in the day, the Sheriff’s Department released a list of nearly 300 people who were unaccounted for. By evening, Honea said the doubling of the number of missing individuals was the result of a comprehensive review of 911 calls or other early reports of people who could not be located. It’s possible many of those people have not been in contact yet with family or friends, he added.

Firefighters made progress on controlling the 141,000-acre wildfire, increasing containment lines to 40 percent Thursday.

The cause of the blaze remains under investigation, and officials have said they’re looking at the possibility that it was sparked by electrical equipment. But Thursday, they revealed that they have identified a possible second origin of the blaze near Concow. They provided no additional details.

Stable conditions and light winds allowing Camp Fire smoke to bring down visibility readings this morning. #cawx pic.twitter.com/nzOX3cVsAM — NWS Sacramento (@NWSSacramento) November 15, 2018

Meanwhile, officials said that more than a dozen patients — including three firefighters — have been treated at Northern California burn centers in the days after the fire that started Nov. 8 leveled Paradise and surrounding Butte County communities.

In addition, officials were alerted that President Trump planned to visit the disaster area Saturday to meet with “individuals impacted by the wildfires,” according to a White House statement.

On Thursday, officials at the Oroville headquarters of the Sheriff’s Department began collecting DNA samples from people who may have lost loved ones.

Several families arrived in the morning to submit cheek-swab samples to help authorities possibly identify their loved ones. The samples are needed because, in many cases, fire victims were burned beyond recognition. The county has brought in a Colorado company, ANDE, to expedite the process.

Honea said that of the 63 victims, authorities have tentatively identified 53 but are waiting for DNA results to confirm who they are.

ANDE’s technology, which looks like a large microwave oven, can analyze DNA within about two hours, a lot faster than older methods. The FBI authorized the company’s technique as part of the Rapid DNA Act of 2017 for use in solving crimes or assisting in war zones. This is the first time the firm has been called to a natural disaster, said spokeswoman Annette Mattern.

“We’re just honored we could in some small way be helpful,” Mattern said.

The search and recovery effort is a huge undertaking. It includes nearly 500 crew members who have been sifting through debris for signs of remains.

In Paradise and the neighboring community of Magalia, search workers wearing white hazardous-materials suits and face masks and carrying rakes scoured properties Thursday. Toward the center of each town, crews worked furiously to bring evacuated residents one step closer to returning to their properties to assess the damage.

But there is still a great deal to do before authorities can consider repopulating the fire zone and opening the checkpoints, officials said.

Melted power lines must be replaced, along with wooden utility poles that are now broken into pieces, some suspended from wires. Trees that burned from the inside out threaten to crash to the ground.

“People can’t come into these areas until the power lines and the trees are rendered safe,” said Capt. Chris Vestal of the California Department of Forestry and Fire Protection. “Typically repopulation is allowed in phases. Once an area is out of danger, people can go back.”

Pacific Gas and Electric Co. workers were out in force replacing power equipment as authorities installed stop signs to replace ruined stoplights. Piles of debris were still smoking around the towns.

Complicating recovery efforts Thursday was an officer-involved, fatal shooting in the evacuation zone near Paradise. A man wanted in connection with two homicides in 2014 was located in a parking lot and subsequently led authorities on a car chase that ultimately ended with the suspect shot and killed after he appeared to point a metallic object at officers, according to Butte County District Attorney Mike Ramsey.

A Sutter County sheriff police dog named Bandit was killed in the encounter, as was a pit bull that was with the suspect.

As of Thursday, the blaze had destroyed 11,862 structures, including 9.700 homes, Cal Fire said.

Smoke from the wildfire continued to plague Northern California, closing down schools across much of the Bay Area as well as cable car lines in San Francisco.

Chronicle staff writers Nanette Asimov and Evan Sernoffsky contributed to this report.

Sarah Ravani, Jill Tucker and Gwendolyn Wu are San Francisco Chronicle staff writers. Email: sravani@sfchronicle.com, jtucker@sfchronicle.com, gwendolyn.wu@sfchronicle.com Twitter: Twitter: @sarravani, @jilltucker, @gwendolynawu