A wildfire burning in the canyons and steep mountainsides near Boulder became one of the most destructive blazes in Colorado history Wednesday as authorities determined it had destroyed at least 135 homes in just three days.

The dire assessment came on the same day firefighters were able to contain about 10 percent of the blaze that has scorched about 6,200 acres, or roughly 10 square miles. It was the first time officials reported being able to hold any part of the 20-mile-long fire perimeter.

Firefighters encountered a tangle of rattlesnakes, downed power lines and combustible propane tanks and struggled to get an upper hand on the inferno.

The Boulder County sheriff's office said Wednesday that 135 homes have been destroyed — a toll likely to rise as the blaze rages on and firefighters get a clearer picture of the damage.

About 3,500 people have been evacuated from about 1,000 homes stemming from a fire that broke out in a parched area near of Boulder on Monday. Residents of four neighborhoods will be allowed to return home Thursday morning. It wasn't clear how many homes that involves.

Four people remain missing as some residents have stayed behind and risked their lives to try to save their homes.

No deaths or injuries have been reported at this point, and the cause of the fire was not known.

The fire west of Boulder is not large in terms of size, but it struck in a populated area that inflicted major property damage.

The reported loss of homes surpasses that of the 2002 Hayman fire in southern Colorado that was the most destructive in the state's history. That fire destroyed 133 homes and 466 outbuildings over 138,000 acres in a more sparsely populated area that includes national forest land.

Todd Richardson, incident commander, told a crowd of about 600 at a public meeting Wednesday night in Boulder that it has cost $2.1 millon so far to fight the fire.

Firefighters took advantage of cooler temperatures and light rain to attack the wildfire Wednesday but authorities acknowledged they still don't have a good handle of the blaze. Fire officials scheduled a meeting night on the University of Colorado campus to update the community.

"We just don't have control of the fire," Sheriff's Cmdr. Rick Brough said Wednesday afternoon as some rain began falling over the fire area.

Air tankers dumped 35,000 gallons of fire retardant on the blaze and crews began building containment lines on the eastern side of the fire. The large plume of smoke the fire had been producing since it started has dissipated because of the favorable weather. However, the fire was still actively burning and threatening structures, forcing some deputies doing an inventory of the damage to retreat.

Fire managers said as many as 500 firefighters and support personnel are at the fire and more are on the way. Laura McConnell, spokeswoman for the management team, said crews are dealing with downed power lines, debris, poison ivy and rattlesnakes. They also have to be watchful for propane tanks in the area.

Brough said the conditions make it too dangerous for anxious residents to return to check on their homes. The fire has also destroyed at least four outbuildings and damaged at least 12 homes, according to a list released by the Boulder County sheriff's office.

At the Colorado Mountain Ranch, 60-year-old Mike Walker has been making a stand against the fire with his wife and 25-year-old daughter in a desperate effort to save the children's summer camp and outdoor recreation center they operate.

"He's safe, he's up there," said Walker's 19-year-old daughter Rose, who evacuated. "He just won't leave. We never doubted where he was, he just won't leave for anybody."

Rose Walker said her father, mother and sister are trying everything to save their ranch, with her father using a tractor to scoop up flames away from structures, "literally dragging the fire away from the buildings." On Wednesday, Rose Walker said her family were still at their ranch, using rakes and backpacks filled with water and a hose to put out any hot spots.

Fire conditions were expected to worsen Thursday night into Friday and the risk of any new fires quickly spreading was high along the populated Front Range region, according to the National Weather Service.

Seven of the country's 19 heavy air tankers have been sent to Colorado to fight the blaze, considered the nation's top firefighting priority. Two more have been dispatched to the fire, said Ken Frederick, spokesman for the National Interagency Fire Center in Boise, Idaho.

Families like the Walkers have been carrying out their own fight against the fire.

Firefighters have been supplying them with water when they can and Rose Walker said she's been crossing into the fire zone to bring her family food and supplies, although authorities have been reluctant to let her come up to the ranch.

Despite her family's efforts, 35 structures have burned, including the family's home, sheds, barns and work areas, Rose Walker said. It's not clear if those are among the total structures that authorities have already confirmed have burned.

"It's everything to us. It's home, it's our work, it's our life," Rose Walker said.

She said family friends have started a Facebook page for the family to encourage people to make donations to help with supplies, food and help replace the tools her father has lost in the fire.

Brough said authorities don't have the time or manpower to force people to leave. However, he said that if a missing person is linked to a burned home, authorities will have to go to the home to see if there are any human remains, tying up resources.

"People are going through trying times right now. We don't have the resources to go up and arrest everybody that's not leaving the area," Brough said.

Meanwhile, those who abided by the evacuation order were frustrated that they couldn't do more to help.

William Bradshaw has grown restless watching the smoke plume over Boulder as he stays in a shelter at the YMCA.

"I don't know if my house has burnt to the ground but not just my house, but all the precious things that I have accumulated in my lifetime," said Bradshaw.

The belongings left behind include the ashes and fingerprints of his son who died at 16.

Resident Dan Hackett prepared to hike two miles from a roadblock to check on the condition of his home. He hiked in once before and found it was still standing. A house 200 feet away was lost and foundations and metal fixtures like sinks was all that remained of houses in some spots, he said.

The fire center also dispatched hot-shot crews to Colorado — teams which Frederick called the "Marine Corps of firefighters."

"If there's any good news, it's that we're at the tail end of the fire season nationally and there's a good availability of resources," Frederick said.

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Associated Press writer Ivan Moreno contributed to this report.