Story highlights "We're gaining," U.S. Forest Service incident commander says

Families may be allowed to return to some neighborhoods Wednesday and Thursday

The fire has scorched at least 46,600 acres in High Park, Colorado

Thick clouds of smoke are visible from miles around; 1,000 personnel now involved in fight

Hundreds of residents ordered out of their homes as a massive wildfire advanced on the suburbs of Fort Collins, Colorado, may be allowed to return Wednesday evening, fire officials said.

"We're gaining," said Bill Hahnenberg, the U.S. Forest Service's commander for the team battling the High Park wildfire, which has burned 46,600 acres in northern Colorado. The fire, which has claimed at least one life, is estimated to be 10% contained.

About 100 structures are confirmed lost, but hundreds of families are anxiously waiting to hear whether their homes survived.

"This fire's behavior is starting to diminish, at least in some places, where we can have trained individuals to go in and determine which structures are lost," Hahnenberg said during his Wednesday morning media briefing.

Hundreds of firefighters have arrived from around the United States to help local departments that have been battling the fire, bringing to 1,000 the number of personnel involved in the fight.

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The local firefighters, some of whom have lost their own homes, are tired, Hahnenberg said. "We're doing our best and have enough resources on board to rotate in and relieve them."

They've had "some success, some failure" in containing the advance of flames eastward and southward, toward residential areas, he said.

Residents should be able to return to two evacuated neighborhoods later Wednesday and perhaps three areas Thursday, said Nick Christensen of the Larimer County Sheriff's Department.

Mark Engle's family decided not to leave their Colorado home despite the thick smoke billowing through the air outside Tuesday. From a window, he watched deer grazing in his backyard, driven out of the forest by flames that have devoured thousands of acres of land only a few miles away. His children's backpacks were placed by the door, stuffed with their favorite toys.

His family was packed and ready to leave in a hurry, Engle said.

But they wanted to stay put, even though authorities have ordered residents in the area to clear out.

"There's a number of people like myself, (for whom) packing up and leaving when you have livestock and animals just isn't as easy as if you have just a house," Engle said.

First measured at two acres early Saturday, the High Park Fire -- which officials say was caused by lightning -- has grown exponentially in the time since.

President Barack Obama telephoned Colorado Gov. John Hickenlooper on Tuesday and said his administration is already making personnel, equipment and federal grants available to the state to help in the effort.

The huge plumes of smoke from the High Park fire are visible from miles away, casting a somber shadow even over communities well outside the danger zone.

CNN iReporter Dave Thrush, a musician from Denver, posted dramatic pictures of smoke darkening the skies above the Colorado State University campus Sunday.

"The fire is 20 miles from the campus, so no one is in danger there, but the billowing smoke is still very eerie," he wrote.

Another CNN iReporter, goodline69, a restaurant server in Lafayette, Colorado, told CNN on Monday: "It was a beautiful day in Fort Collins with business as usual for many, it seemed, so it was a bit surreal to be watching homes being destroyed from across the reservoir.

"I found myself watching the crowd watch the fire. Heartbreaking."