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By Alex Johnson

The number of people unaccounted for in the Camp Fire zone of Northern California more than doubled on Thursday as authorities said the death toll in the deadliest wildfire in California history rose by seven, to 63.

Butte County Sheriff Kory Honea, who provided the updated figures at a news conference on Thursday night, said information continued to come in rapidly and was likely to change at any time.

The Camp Fire, which ignited Nov. 8 in the town of Pulga, continued to grow slowly, reaching 141,000 acres on Thursday, with containment reported at 40 percent. Officials say the fire is not expected to be fully contained until the end of November.

The number of missing people continues to skyrocket, rising from 297 on Wednesday night to 631 on Thursday night, Honea said.

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With authorities working to synthesize reports from three separate missing-persons hotlines, "you have to understand, folks: This is a dynamic list," Honea said. "Some days there might be more people. Some days there might be less people."

Honea said it was likely that "a lot of people don't know that we are looking for them."

The number of structures that have been destroyed stood at 11,862, including 9,700 single-family homes — about 1,000 more than were reported on Wednesday.

Authorities said they had no firm accounting of how many people had been driven from their homes into shelters. But they said that at its height earlier in the week, the number reached 52,000.

Some areas saw evacuation orders reduced and warnings lifted on Thursday and utility companies have started restoring essential services with crews working to remove hazardous trees, weakened utility poles and damaged piping.

Some of those evacuees faced a new threat, said health officials, who confirmed that 21 people were ill with norovirus at a shelter at Neighborhood Church in Chico, where 179 people were being housed Thursday. Norovirus is a contagious virus that causes severe vomiting and diarrhea.

Lisa Almaguer, a spokeswoman for the Butte County Public Health Department, told NBC News that lab tests had confirmed the presence of the virus at the shelter. She said the agency was isolating people with symptoms from the rest of the shelter population and that it was "taking all necessary prevention measures" in other shelters, even though no lab-confirmed norovirus had been detected elsewhere.

Offshore winds drove choking smoke into the San Francisco Bay Area, said Kristine Roselius, a spokeswoman for the Bay Area Air Quality Management District. Bay Area public schools planned to close Friday.

No official cause for the fire has been determined, but Pacific Gas & Electric said in a document filed with the U.S. Securities and Exchange Commission this week that its equipment may have sparked the blaze.

The White House said Thursday that President Donald Trump, who pledged the full support of the federal government to Gov. Jerry Brown this week, would visit California on Saturday to assess the damage of both wildfires that have ravaged the state — the Camp Fire and the Woolsey Fire, which has killed three people in Southern California.

The president's visit comes after he was excoriated for publicly scolding state officials and then blamed and threatened Californians, tweeting in part, "so many lives lost, all because of gross mismanagement of the forests."

Yuliya Talmazan contributed.