Smoke fills the sky in the mountains between Four Mile Canyon and Left Hand Canyon, northwest of Boulder, Colo., on Monday. (Kathryn Scott Osler/Denver Post/Associated Press)

Firefighters in Colorado are planning to ramp up an aggressive fight against a 1,416-hectare wildfire that by Tuesday had forced thousands of people from their homes in a rugged canyon northwest of Boulder.

With calmer winds in the forecast, authorities said they planned to dump large amounts of fire retardant from the air. Gusty winds Monday grounded air tankers for much of the day.

The fire has destroyed dozens of homes, and around 3,000 people have been forced to flee the area.

At a news conference Tuesday, Rich Brough of the Boulder County Sheriff's Office had no details on precisely how many homes have been damaged or destroyed.

Though the cause of the fire isn't known, Brough said there's no indication it was set intentionally.

Fire 'more active' than anticipated

The blaze broke out Monday morning in Four Mile Canyon northwest of Boulder and rapidly spread across roughly 1,400 hectares. Erratic wind gusts sometimes sent the fire in two directions at once.

Kurt Rieder and his nine-year-old daughter, Lily, watch the smoke from a wildfire burning in the Four Mile Canyon area just west of Boulder, Colo., on Monday. (Peter M. Fredin/Associated Press)

Crews managed to save the historic town of Gold Hill, including an Old West grocery store and structures once used for stagecoach stops. But firefighters in the area had to relocate their engines and equipment several times to avoid the flames.

"The fire moved too quickly and was much more active than anticipated," Haberstick said.

More than 100 firefighters were on the scene Monday, and the winds quieted enough by late afternoon to allow three tankers to drop fire retardant along the leading edge of the fire.

The winds pushed the fire through three canyons where pine trees have been left prone to fire by disease, drought and beetles that burrow under the bark of pine trees, fire managers said.

"We haven't had any rain there for almost a month, maybe more than a month," said Craig Douglas, who lives north of the fire and received a knock on the door from a sheriff's deputy at about 8 p.m. local time on Monday.

"The humidity the last couple of days has been in the single digits, so it was a fire waiting to happen."