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(CNN) -- A sprawling wildfire in northern Colorado grew larger than the nearby city of Fort Collins on Monday, racing swiftly across a crackling dry landscape, spitting flames as far as 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.
"The hope for containment today, I will tell you, is tenuous," incident commander Bill Hahnenberg said.
The fire nearly doubled in size overnight to 36,930 acres, or more than 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 zone 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 more than 170 animals dropped off by evacuees.
The fire was visible from the Colorado State University campus in Fort Collins, where students and staff were told to move activities inside if possible. The university's Foothills campus was serving as command center for the firefighting efforts.
The fire is "very large, unpredictable, aggressive," Christensen said.
It was moving fast, 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.
More than 100 structures had been damaged or destroyed, Sheriff Justin Smith said Monday. He did not know if they were homes, sheds or other buildings.
Incident commanders said the fire is highly likely to continue to grow, but no further evacuations were currently on tap.
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.
Marty Schriefer, whose 42-acre property lies within the Roosevelt National Forrest, has kept an eye on the fire map since evacuating Sunday afternoon.
"Looking at the maps today, I'm pretty sure our house is gone," he said Monday.
His wife called him at work Saturday morning to tell him she could see the looming smoke.
"We spent Saturday afternoon collecting up valuables, paperwork, irreplaceable stuff -- photos, stuff like that," he said.
Things looked promising through most of the weekend, Shriefer said, but when the fire suddenly came within an hour of his home Sunday, he knew it was time to go.
"It exploded, really," he said. "Very quickly at about 2:00 huge black plumes went up from that area."
The couple is staying in his wife's office in Fort Collins with their dog and cat they were able to rescue. They were forced to leave two other cats behind.
"It was tough to leave them," he said. "But we couldn't catch them and we had to get out. We left food and water in the house and left the door open for them."
Still others were chancing the risks and staying at home.
Grant Campbell of Laporte 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."
Campbell said it would take an "apocalypic event" to get him to leave.
"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.
CNN iReporter David J. Thrush watched a dramatic sunset Sunday through the smoke cloud 40 miles away in Eaton, Colorado.
"It was an eerie scene," he said. "The sunset was a blazing ball of red, almost apocalyptic, through the smoke.
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.
The fire was one of the highest priorities for fire managers nationwide, Hahnenberg said. Five of the nation's nine heavy air tankers had been assigned to the fight, Larimer County officials said.
Hahnenberg expects the number of firefighters assigned to the team to grow to 500 or 600 by Tuesday.
Five smaller tankers, some from Canada, and seven helicopters were also working the fire, authorities said.
Hahnenberg's 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.
The cause of the fire is under investigation, but authorities believe lightning is to blame.
First measured at two acres early Saturday, the High Park Fire has grown exponentially in the time since -- including more than doubling in size Sunday and again overnight into Monday.
CNN's Greg Morrison and Chelsea J. Carter contributed to this report.