(CNN) -- A raging, fast-moving Colorado wildfire continued to grow Sunday, prompting evacuations as some 20,000 acres burned, authorities said.
First measured at two acres early Saturday morning, High Park fire has grown exponentially in the time since -- including more than doubling in size over the course of Sunday.
Firefighters' primary goal is to get people out of harm's way and try to save as many buildings as possible, said Larimer County Sheriff Justin Smith said. Still, he admitted much is still beyond authorities' control, thanks to low humidity, high temperatures and dry brush fueling the flames.
"This thing has a mind of its own, and it's pretty much telling us what to do," Poudre Fire Authority Chief Tom Demint told reporters Sunday night.
Calls have gone out to around 2,000 phone lines attached to homes around the area, Larimer County Sheriff's Office spokesman Nick Christensen said.
The location and rate of evacuation requests -- which have come almost continuously over a 36-hour stretch, and on all edges of the blaze -- "is completely unheard of," according to Smith.
Responding to "rumors" that several people have died because of the fire, the sheriff said only that "we have a single person we still can't account for, in a location we believe somebody could have burned."
He also said authorities are looking into a report that two hikers were missing in the area of the blaze.
InciWeb, the U.S. multi-agency Incident Fire Response website, confirms 18 structures have been lost or damaged. However, Smith concedes that number may be higher given the difficulty in reaching and assessing some areas, adding he doesn't anticipate a firm count on damaged homes "in the near future."
Smith lauded the work of firefighters, saying he's been struck seeing "several structures surrounded by black that were still standing," indicating the fire had gone all around but left them untouched.
Crews have had some success safeguarding some homes and businesses. Yet the sheriff said "hundreds of homes" need protecting, ceding some may not be able to be saved.
While temperatures had cooled somewhat, low humidity and sustained winds of 15 mph to 20 mph -- and gusts as strong as 40 mph -- continued to fan the blaze Sunday.
The wildfire's erratic nature is complicating efforts: It is not just moving in one direction and, in some cases, is returning to scorch areas it had already burned, according to the sheriff.
"This fire is just running crazy," Smith said.
There has been no definitive determination as to the conflagration's cause, though authorities so far "have not seen any indication of a man-started fire," Smith said.
At least 250 firefighters, along with air tankers and helicopters, were spearheading the fire-control effort on the ground and in the air, officials said.
Plus, sheriff's office employees and local firefighters have gone door-to-door urging people to leave, with members of a wedding party among those forced out. Some evacuees have taken shelter at a middle school in La Porte.
A shortage of equipment and manpower was compounding the problem, though authorities said they hoped to remedy that. Regional resources are already deployed fighting other outbreaks, such as the Whitewater Baldy fire in New Mexico.
"(The fire) is so out of control, developing a plan is difficult until we get the resources here," said Demint, noting the dozens of fire engines deployed can only do so much in corralling the charging blaze. "You do the math and you see what kind of (trouble) we're in."
CNN's Greg Morrison and Deanna Proeller contributed to this report.