A sprawling wildfire in northern Colorado grew larger than the nearby city of Fort Collins on Monday, racing across a crackling dry landscape at up to 40 feet a second, spitting flames up to 300 feet into the air and forcing thousands out of their homes.
The Red Cross, Humane Society and other aid groups mobilized to help evacuees while at least 400 firefighters, aided by air tankers and helicopters from as far away as Canada battled the fire about 15 miles west of Fort Collins.
"Yesterday when the winds had shifted to northerly winds we could see this massive, most amazing smoke cloud that looked almost like a volcanic eruption," said Grant Campbell, a Laporte, Colorado resident who is staying in his home despite an evacuation order issued Sunday.
"It really is beautiful in a kind of surrealistic way, but you also have to remember that it puts people and their homes in danger," he said.
The fire nearly doubled in size overnight to 36,930 acres, or 57 square miles, Larimer County Sheriff's Office Executive Officer Nick Christensen said Monday. It had been estimated at 20,000 acres Sunday night.
The rapid growth made the fire larger than Fort Collins, which is 47 square miles.
While Fort Collins was not immediately threatened by the fire, a smoky pall hung over the city of 143,000, said Stephanie Ashley, a spokeswoman for the Larimer Humane Society.
"It's pretty much a haze covering the town, and you can definitely smell it," said Ashley, whose shelter was housing 170 animals dropped off by evacuees.
The fire is "very large, unpredictable, aggressive," Christensen said.
It was moving at 20 to 40 feet per second, or about 1 mile an hour -- fast for a fire, Christensen said. Flames of 15 to 20 feet were commonplace, with some reaching 300 feet, he said.
While lower temperatures, slightly calmer winds and higher humidity were offering modest help to firefighters, the fire was entirely uncontrolled Monday morning, he said.
Incident commanders said the fire is highly likely to continue to grow, but no further evacuations were currently on tap, Christensen said.
A number of homes have been lost, Christensen said. He could not provide specifics.
Crews worked around the clock to evacuate residents, in some cases as "flames were licking at the units that were doing the evacuations," Larimer Sheriff Justin Smith said Sunday.
"We have planned and trained for fires in every neighborhood. But this fire hit every neighborhood at once," Smith told reporters.
Although the exact number of people under evacuation orders was not known Monday, authorities had put out more than 2,600 calls notifying residents of the evacuation orders, Christensen said.
He urged residents to heed to orders, saying fire crews cannot afford to spend too long in any one location.
Campbell said it would take an "apocalypic event" to get him to leave. He has lived in the same house for 22 years and has been ordered to leave numerous times over the years but never had any damage.
He's only followed the orders once, and now he's used to living through fires in the area.
"If we walk outside, it's just like being next to a campfire," he said. "It's smoky in the house, too. You get used to it in your home, so you just don't smell it anymore."
Only one injury has been attributed to the fire, heat exhaustion suffered by a firefighter on Sunday, Christensen said.
However, one person was listed as missing, Christensen said.
Five heavy air tankers and numerous lighter aircraft, including some from Canada and from the Colorado National Guard, were helping fight the fire Monday, authorities said.
While regional resources were already taxed fighting other outbreaks, such as the massive Whitewater Baldy fire in New Mexico, more assistance was still needed, Christensen said.
A Type 1 incident response team -- the most advanced and capable available for a wildfire -- began working the Colorado fire Sunday night, according to Larimer County authorities. Additional crews had been ordered, authorities said.