Hundreds of people evacuated during one of Colorado's most destructive wildfires returned to their scorched homes Sunday, just as residents about 35 miles to the northeast fled houses threatened by another fast-moving blaze.

By sundown, the second fire had destroyed at least one Loveland-area home while threatening other structures. It burned about 600 acres, or just under a square mile, forcing authorities to pull some resources from the Boulder fire.

Meanwhile, firefighters in Boulder — surrounded by the dreary sight of burnt trees, burned-out vehicles and uneven patches of blackened ground — inched closer to fully containing the blaze that has burned 10 square miles. Authorities continued to investigate what sparked the fire.

A senior law enforcement official familiar with the investigation told The Denver Post that authorities are looking into whether a fire pit was the cause, 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.

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 is 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.

At one destroyed property, all that remained was a stone chimney surrounded by walls of brick about waist high. Saplings in the front yard were burnt and barely their trunks remained. A barbecue grill lay upside down, along with seven metal mailboxes nearby. The house's separate garage had been reduced to a heap of ashes.

As residents returned home, some pleaded for privacy from the media.

Utility workers were restoring electricity to the homes where residents had returned, repairing and installing new poles and lines. Qwest Communications also was working to fix about 3,000 telephone and DSL lines.

Fire officials warned that much of the area is dangerous because of downed power lines and poles, damaged roads and exposed mine shafts.

Still, Boulder firefighting operations were being scaled back and some crews were being relieved six days after the wildfire erupted and quickly destroyed at least 166 homes. The blaze was 73 percent contained Sunday and officials said full containment was expected by Monday evening.

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.

"That's probably one of the hardest walls for all the firefighters to go up to," Krasko said. "They really have a tough time with that. The community support has been tremendous for them."

So far, the fire has cost more than $6.7 million to contain. Winds drove the fire out of control and strong gusts later in the week triggered fears that it might spread into the city of Boulder. Officials urged residents to prepare to evacuate, but fire lines held and no evacuations were needed in the city.

The Boulder Sheriff's Office is leading the investigation into the cause and origin of the fire, with assistance from the U.S. Forest Service. The loss of homes surpassed that of the 2002 Hayman fire in southern Colorado, which destroyed 133 homes and 466 outbuildings over 138,000 acres, or more than 215 square miles.

That fire was started at a campground by a U.S. Forest Service employee who said she had burned a letter from her estranged husband. She served six years in prison for it and was released in 2008.

Insurers had no immediate damage estimate for the Boulder neighborhoods filled with a mix of million-dollar homes and more modest log homes and ranches.

The Boulder Daily Camera reported the wildfire destroyed at least $76.9 million worth of property, based on a database of buildings confirmed burned and their valuations listed in Boulder County property records.

In Loveland, crews battled the second wildfire, which grew within hours Sunday from 35 acres to 600 acres. In addition to the home that was destroyed, the fire has claimed four outbuildings and an RV, but no injuries have been reported, said Merlin Green, the division chief for Loveland Fire and Rescue. No sections of the fire had been contained by Sunday evening.

Eighty firefighters from six agencies battled the blaze, along with four helicopters and nine air tankers. Authorities haven't determined what caused the fire.

Residents within a four-mile radius of the fire are under a mandatory evacuation order.

Cindy Bragdon, who lives in one of the areas ordered evacuated, said she and her family were returning from Steamboat Lake when her daughter called to let her know about the fire. Bragdon was with about four other families at a Loveland church where the American Red Cross Mile High chapter set up a shelter. There, residents watched televised coverage of the fire on a big screen.

"We were at Steamboat Lake and we were having a perfect weekend until we got back," she said.


Associated Press writer Ivan Moreno contributed to this report.