LOUISVILLE, COLO. -- Investigators seeking the cause of the Colorado wildfire that destroyed nearly 1,000 buildings have narrowed their search to an area near Boulder, but it could be days or weeks before details are released, the sheriff said Monday.

The search is focused on an area where a passer-by captured video of a burning shed on the day the fire began, Boulder County Sheriff Joe Pelle told a news briefing. He said dozens of people have been interviewed thus far.

Experts from the FBI, the Bureau of Alcohol, Tobacco, Firearms and Explosives, and the U.S. Forest Service were participating in the investigation, Pelle said.

The sheriff declined to offer many more details on Monday, a day after saying that the fire "originated somewhere" in the neighborhood with the burning shed.

Declaring that "the stakes are huge," Pelle said he would not comment on the probe until he was ready "to announce some progress -- perhaps that may be a week, perhaps that may be a month."

Getting it right, he said, was "more important than the urge for speed that a lot of folks are feeling right now."

Experts say the winter fire was rare but that similar events will become more common as climate change warms the planet and suburbs grow in fire-prone areas. The inferno broke out unusually late in the year following months of drought that included a dry fall and a winter with hardly any snow so far.

No downed power lines were found in the area being investigated, according to the county's Office of Emergency Management.

Meanwhile, teams continued searching Monday for two people who were still missing, and survivors sorted through the charred remnants of their homes to find whatever was left.

The Boulder County area known as Marshall Mesa is near the base of the Rocky Mountain foothills and overlooks the more heavily populated suburbs to the east that were devastated by the fast-moving fire, which was whipped up by furious winds blowing from the foothills. The area is surrounded by tinder-dry public open space and private grasslands.

Over the weekend, authorities executed a search warrant, but the sheriff declined to elaborate and did not comment on whether he thought the fire was arson.

A sheriff's official who spoke on condition of anonymity confirmed that several properties were under investigation, including one in the Marshall Mesa area, about 2 miles (3.2 kilometres) west of the hard-hit town of Superior. A National Guard Humvee blocked access to the neighborhood on Monday.

In the search for the missing, crews were looking for a woman in the town of Superior and a man from the nearby community of Marshall. Pelle said the crews were sifting debris by hand and using small tools.

Louisville Police Chief Dave Hayes said authorities used cadaver dogs to re-check destroyed properties as a precaution. He said no one was reported missing in the heavily damaged city, but that "doesn't mean we won't find something." Hayes told reporters after the briefing that he lost his own home and was wearing a change of clothes he asked someone to buy for him.

Gov. Jared Polis told the briefing that it was "remarkable that a fire of this speed and size" resulted in only two people missing. Tens of thousands of people evacuated on Thursday, and Polis emphasized the importance of heeding evacuation orders.

"When you get a pre-evac or evacuation notice, hop to it. The residents did, and most of them are with us today," he said.

While homes that burned to the foundations were still smoldering in some places, the blaze was no longer considered an immediate threat -- especially with frigid temperatures and a blanket of snow that fell Saturday.

Most of the 991 buildings destroyed by the fire were homes. But the blaze also burned through eight businesses at a shopping center in Louisville, including a nail salon and a Subway restaurant. In neighboring Superior, 12 businesses were damaged, including a Target, a Chuck E. Cheese pizzeria, a Tesla car dealership, a hotel and the town hall.

The two towns are about 20 miles (30 kilometres) northwest of Denver and have a combined population of 34,000.

Among the homes that were still intact, utility crews went door to door to check if natural gas and electricity could be safely restored.

"Is there toxic fumes? Are we OK to move back in?" asked Nancy Alderson, who said she was worried about plastics and other materials consumed in the blaze.

Over the weekend, authorities distributed thousands of space heaters to families who endured several days of freezing temperatures inside homes spared by the fire.

"What a relief," uttered Louisville resident Carl Johns as a utility worker turned on a gas valve and went inside Johns' home of 21 years to make sure appliances were lighting up. He had been evacuated since Thursday, when police drove through the neighborhood and urged everyone by loudspeaker to leave the area.

Some of his neighbors weren't so lucky. Down the street stood a row of burned homes.

"That just blows me away," Johns said. "The houses aren't there, and you can't recognize your own block."

The Boulder Valley School District, which serves the wildfire area, planned to resume classes as scheduled on Wednesday and to provide counseling services for students and staff affected by the flames. The University of Colorado in Boulder delayed in-person classes to Jan. 24, with remote learning starting Jan. 10.

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