A senior law enforcement official familiar with the investigation told the Denver Post that authorities are looking into whether a fire pit sparked the blaze, which could mean criminal charges are possible. The newspaper did not name the official.
Authorities previously said the fire may have started after a vehicle crashed into a propane tank. The sheriff's office is aware of the Post article but won't comment on the cause or origin of the fire because it's under investigation, said Sarah Huntley, a spokeswoman for the fire response.
Utility workers were restoring electricity to homes where about 2,000 residents have been allowed to return in the rugged foothills above Boulder. Much of the area is dangerous because of downed power lines and poles, damaged roads and exposed mine shafts, officials said.
Firefighting operations were being scaled back and some crews were being relieved six days after the wildfire erupted and quickly destroyed at least 169 homes.
Like other residents, Nancy and Jim Edwards picked up a permit Sunday morning to re-enter their neighborhood, but they found out that the roads leading up to where they live are still closed. Jim Edwards said they might drive as far as they're allowed.
"We might take a ride, but it is really heartbreaking to see the stuff," he said.
Edwards said he spotted their house through a telescope from Flagstaff Mountain outside Boulder and saw that it was destroyed.
"It looked like a nuclear disaster," Nancy Edwards said. She said they plan to rebuild.
Fire spokesman Terry Krasko said Sunday firefighters have been overwhelmed by the community's gratitude and are especially touched by a wall of thank-you notes at their command camp.
Inside the burn area Saturday, crews worked to snuff out smoldering stumps, using shovels, axes and water carried on backpacks. Fire trucks and water tenders ferried water up the mountains and down the canyons while helicopters dropped water on hot spots.
"There is a lot of unburned fuel and a lot of houses at risk," warned Don Ferguson, a spokesman for the incident command.
It was 73 percent contained Saturday night and crews, taking advantage of calmer winds, hoped for full containment by Monday evening. Some 1,000 firefighters from 20 states dug lines and tamped out hot spots.
The fire left some houses standing among blackened forests while homes nearby burned to the ground. Burnt cars littered driveways. At one home, a winding stucco concrete staircase rose about 15 feet into open space - where a house used to be. Beyond, mountains in the distance sprouted 100-acre patches of burned trees surrounded by green forest and untouched homes.
Firefighter Steve Reece spent Saturday day digging out grass and cutting through roots with a tool that's part shovel, part hoe and part ax to snuff out hot spots.
(Left: On Sunday Alex Rivera leads a group of residents from the Justice Center in Boulder, Colo., where they picked up permits from the sheriff's office to allow them to return to their homes in the area of the wildfire burning west of Boulder, Colo.)
It has cost more than $6.7 million to fight the fire, which was quickly fanned by gusting winds. Winds picked up again later in the week, leading to fears that the fire might spread into the city of Boulder. Officials urged residents to prepare to evacuate, but fire lines held and no evacuations were needed.
The loss of homes surpassed that of the 2002 Hayman fire in southern Colorado, which destroyed 133 homes and 466 outbuildings over more than 215 square miles.