LOVELAND, Colo. — Firefighters worked Monday to control a 900-acre wildfire that destroyed at least two homes in the northern Colorado foothills as authorities said the blaze and another one that burned at least 166 homes were sparked by household fires.

Crews were hopeful they could stop the newest fire near Loveland from damaging more houses. They attacked it quickly with aircraft and hundreds of firefighters brought in to fight the week-old wildfire near Boulder.

Containment lines have been built around 20 percent of the second fire, which broke out Sunday near Loveland.

"We're making progress. It's slow progress," said Merlin Green, division chief for Loveland Fire and Rescue.

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Sheriff's Cmdr. Rich Brough said Monday that the larger fire — which scorched at least 10 square miles and has cost $8.3 million to fight — was most likely sparked Sept. 6 by a fire started in a pit days earlier.

A lifelong area resident doused the fire with water and stirred the ashes to put it out, Brough said, but gusty winds brought the embers to life and blew them out of the pit. The Denver Post reported earlier that authorities were looking at that possibility.

Denver's KUSA-TV reported Monday that the Four Mile Fire Department chief said the resident is a longtime volunteer firefighter with the department. The fire chief didn't immediately return a message to The Associated Press.

Sheriff's spokesman Brough wouldn't comment on the report, saying the department hasn't identified the person yet.

The fire was expected to be fully contained by Monday evening. The hope is that most people will be able to return to their homes by Wednesday, Brough said.

Meanwhile, Green said a resident burning a brush pile started the fire near Loveland. It quickly spread in the tinder dry conditions even though winds were much calmer than a week ago.

Residents within a four-mile radius of the Loveland fire — an area with an estimated 200 structures — were told to evacuate. City spokesman Andy Hiller said evacuation notification calls went to more than 1,700 phone numbers.

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Helicopters assigned to the Boulder fire were brought in to drop bucket loads of water Sunday. Air tankers also helped dropped fire retardant to slow the spread of the blaze, allowing fire crews to get in and build containment line.

Air tankers and helicopters returned Monday, with the choppers filling their buckets in a nearby reservoir.

Ron and Carol Christensen confirmed that the fire destroyed their home on Turkey Walk Trail, according to the Loveland Reporter-Herald. The Larimer Humane Society was able to rescue their sheltie.

Numerous cars and vehicles also were destroyed and an unknown number of horses, sheep and other livestock had to be left behind.

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After evacuating Sunday, Tom and Jan Theilgaard looked at the hills with their spotting scope and watched the flames and black smoke above them. They saw their neighbor's home burn but they saw their house was still standing. They had to leave without their horse, who has room to move around on 10 acres of pasture.

Claudine Busleta and her daughter Julia Garcia live in an area on standby for evacuations, but they didn't wait for things to get worse. They evacuated Sunday with four children and grandchildren as soon as they smelled smoke and saw the haze.

"There's only one way down the hill. When we see smoke, we don't wait for a 911 call," Busleta said Monday.

About 300 firefighters — including elite hot shot crews — were expected to be working on the fire by the end of the day.

The Red Cross said 74 people have registered at an emergency fire shelter.