In a faint haze of gray smoke, residents on the outskirts of Denver packed clothing, family photos and even a wedding dress Tuesday in case they had to flee the largest wildfire in Colorado's 126-year history.

The wind-whipped, 87,000-acre blaze stretched for 15 miles along the Rockies foothills. The fire has destroyed 21 homes and threatened another 2,500, said fire information officer Joe Colwell.

Chris and Lori Sutton awoke at dawn to the smell of smoke drifting through an open bedroom window. Chris Sutton said the smoke in his hilly subdivision was so thick it was "like fog," though it blew away a few hours later.

"You're not sure what to do. Do you stay? Go?" he asked, standing outside his home 23 miles southwest of Denver. His wife left for work in a car stuffed with a handful of their belongings. The fire was still 10 miles away but getting closer.

By evening, the fire was moving slowly to the northeast, within 35 miles of Denver. Flames were about seven miles from Roxborough, a small town on the far southwest edge of the metropolitan area.

Hundreds of residents have left their homes, and Douglas County authorities urged the Suttons and more than 13,000 others near Sedalia — 20 miles south of Denver — to leave. Thousands more were told they may have to flee.

Colwell said 400 people had been cleared out of their homes in Teller County. He also said crews were pulled off the fire's southern lines as a safety measure.

"Where it's kicking up is down on the southeast flank," he said. "It's really been creating havoc."

The fire was one of at least eight burning across Colorado, including a 10,600-acre blaze that destroyed 28 homes near Glenwood Springs, about 150 miles west of Denver. That fire was only 5 percent contained, but thousands of people were allowed to return to their homes.

The blaze southwest of Denver was nowhere close to being contained and officials said it was too dangerous to put firefighters on its northern fringes — between the flames and homes in Douglas County, one of the fastest-growing in the country.

"There is such a tremendous amount of heat that you can't put firefighters on the ground in front of it," fire information officer Tony Diffenbaugh said.

As the fire steadily moved northeast, it also grew on its bottom eastern side, U.S. Forest Service spokeswoman Barb Masinton said.

Smoke forced the closure Tuesday of the Florissant Fossil Beds National Monument, superintendent Jean Rodeck said.

In the fire's wake, skeletal trees stood among blackened pine needles and cones on the forest floor. Flames jumped from treetop to treetop as thick, tall plumes of smoke billowed above.

Earlier Tuesday, shifting wind had helped slow the larger fire's march toward Denver and clear an otherworldly haze that had blanketed the city for three days. The haze was the worst that Steven Arnold of the state Health Department's air pollution control division could recall.

"I don't know of another situation where we've had this much smoke emission that you could associate with a single fire," he said.

The fire was started Saturday by an illegal campfire. It exploded over the next day to become the largest fire ever in the state.

"This is way beyond anything we've seen in the last 30 years or so," said Bob Sturtevant, education director of the state Forest Service. "It's a combination of the drought and the buildup of fuels we have in the forest right now."

Wayne Baker, a 17-year fire management officer in the Pike-San Isabel National Forest, said it was the worst fire he has ever seen.

"It's just very scary and way bigger than any of us ever thought a fire could spread in this county," he said. "Everything came together on this fire — the dry fuel, the high wind, everything lined up to make it a big fire."

At least one structure has been destroyed. Up to 300 firefighters, aided by aircraft dropping retardant and water, were battling the blaze.

The fire was being anxiously followed by the Suttons and other residents in Roxborough. Deanne Kirby, 33, her husband and two young sons left Monday to stay with a sister-in-law in nearby Littleton. She returned home a day later to gather additional belongings.

"We just finally got tired of waiting and it was kind of nerve-racking," she said.

Neighbors Anne and Ray Najera crammed belongings into the trunk and back seat of their two-door Honda Civic, including medical and legal records, clothing, medication and Anne's wedding dress.

"I'm pretty stressed out," said Anne Najera, 31. "It's just moving so fast. I did not sleep well at all.

"Basically, we went through yesterday thinking of things we can't replace and packing them," she said. "It's just not knowing. It's driving us nuts."

Near Glenwood Springs, cooler temperatures and lighter winds helped quell the fire and thousands of people were allowed to return home — with a warning that they should be prepared to flee at a moment's notice. The fire broke out on Storm King Mountain, where 14 firefighters died in a 1994 fire.

Elsewhere:

— California firefighters worked in steep, rugged terrain near the Oregon line to combat a 600-acre wildfire that threatened homes and animals Tuesday.

— New Mexico firefighters battling flames on 96,000 acres were helped by calmer winds and rising humidity.