John Bacon, USA TODAY, and Amanda Wright, KUSA-TV

BOULDER, Colo. — Residents of almost 500 homes forced to flee a wildfire burning in mountains within a mile of downtown were allowed to return home Monday, authorities said.

Boulder County Sheriff's Cmdr. Mike Wagner said the fire remained 50% contained Monday, but firefighters had essentially surround the fire, which was burning less than 100 acres. He warned that efforts to douse the blaze could be affected by weather conditions.

"One of the firefighters said it feels like June up there," Wagner said. "There is no moisture."

Wagner called the dry conditions "concerning" and said they could contribute to a major fire season this summer.

"June is a more typical fire season, hotter and drier," he said. "But around here it seems that fire season goes all year long."

Wildfire near Boulder, Colo., forces evacuations

Wagner said the fire may have been human-caused. He said investigators had ruled out a lightning strike or downed power lines.

"I know there are transient camps up there," he said.

Wagner said the two Chinook and two Black Hawk helicopters that battled the blaze Sunday were giving way to a small, more maneuverable helicopter Monday. About 180 firefighters were working the fire, he said.

On Sunday, residents in some neighborhoods gathered on streets and watched the Sunshine Fire burning Sunshine Canyon, an area known for camping, biking, hiking and rustic, expensive homes.

More than 1,000 homes were evacuated early Sunday, but many residents were allowed to return later in the day. The Boulder Office of Emergency Management said no homes or other structures were immediately believed to have been lost. In 2010, a wildfire destroyed nearly 200 homes in the mountainous area west of the city, home to the University of Colorado, Boulder.

The East Boulder Community Center was serving as an evacuation shelter Sunday night. Anne Spalding told the Boulder Daily Camera she was staying with her sister Sunday after being forced to evacuate. She said she was "optimistic" that her home would be spared.

"It's really close to my house," Spalding said. "We didn't even get a call. We had firefighters knocking on the door. They are doing a fabulous job. It's a little scary. We put our hearts into our homes."

Wright reported from Colorado, Bacon from McLean, Va.