(CNN) -- A brush fire that had scorched some 700 acres in north-central Colorado had burned two houses and was threatening dozens more, an official said Monday.
"We're trying to get air resources in there as quickly as we can," said Maj. Bill Nelson, incident commander with the Larimer County Sheriff's Office, about the fire, located west of Loveland.
The fire, he said, was no more than 10 percent contained.
He acknowledged the estimate of the size of the fire -- which was in its second day -- could be off. "It's still real hazy, and we don't have a good edge on the fire," he told reporters. "It's burned very sporadically, a lot of fingers."
"At least" another four or five outbuildings, several vehicles and several trailers in the fire's path have been incinerated, Nelson said.
"We should be able to get a handle on this, I hope, very quickly," he added. "Now, what that 'very quickly' is is still a few days."
None of the residents of the more than 50 houses in the evacuation area would be able to return Monday, he said, adding that no injuries had been reported. Some of those houses were within a half mile of the blaze, he said. "If the fire continues moving, they're in danger," he said. But winds Monday were quiescent.
In all, 250 to 300 firefighters were in the field or managing the efforts in the rocky, steep and heavily timbered property, Nelson said.
At least one air tanker and four or five helicopters capable of carrying heavy loads were available to fight the fire, whose cause was under investigation. "We know it wasn't lightning; we believe it is probably some sort of a man cause," Nelson said.
More tankers were available, but had not been called in because the area affected is small. "We had a hard enough time yesterday keeping them from crashing into each other," he said.
The fire quickly grew from 40 acres to between 600 and 700 acres by Sunday evening between Flatiron Reservoir and Pinewood Reservoir, the city said Sunday night.
The Loveland fire broke out as residents near another Colorado wildfire in Boulder -- 40 miles to the south -- began returning home.
Nelson described Boulder's fire as a stroke of luck for firefighters in Loveland.
"This time of the year, without -- unfortunately -- Boulder also having a fire, we would never have had the response we had today," he said. "Normally this time of the year, it's the off season, the resources typically aren't here."
Emergency management officials in Boulder said the Fourmile Canyon fire was 100-percent contained late Monday.
The massive wildfire appears to have been sparked by a fire pit at a residence, said Boulder County Sheriff's Office Cmdr. Rick Brough. He added the property owner had made attempts to extinguish the fire by dousing it with water and stirring the ashes, but that the wind probably reignited the embers and blew them out of the pit.
Brough said it was still too early to say whether criminal charges would be pursued.
Several thousand residents were evacuated at the height of the fire, but were being allowed to return Monday as firefighters mopped up hot spots, according to Boulder County emergency management.
The Boulder fire, which started early last week, charred more than 6,400 acres of mountainous countryside and destroyed 172 structures -- 166 of which were homes -- on Boulder's far west side.
CNN's Justin Lear contributed to this report.