Wind-driven wildfires swept across Boulder County grasslands Wednesday, destroying at least four structures and prompting mandatory evacuations of almost 900 homes.

Officials said late Wednesday that 391 new homes were being evacuated west of the Boulder Heights neighborhood, in addition to at least 500 homes on the north side of Boulder that were evacuated earlier in the day.

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Sheriff's Cmdr. Phil West said two firefighters suffered minor injuries.

Authorities said more than 11,000 homes had been alerted to the danger by reverse-911 calls, but not all of those households were told to leave.

"At this point we don't anticipate rescinding the evacuation order until sometime tomorrow morning," West told KDVR-TV. "There's still too many hot spots, too many places where the fire could flare up again if the wind changes."

The flames — part of three wind-driven blazes — started in parched, rolling grasslands dotted with subdivisions, individual homes and horse ranches about 25 miles northwest of Denver.

Authorities said at least three of the destroyed structures were homes, and the fourth was either a barn or a home.

The largest fire, which burned more than 10 square miles, jumped across U.S. 36 and entered the Lake Valley Estates neighborhood, where police went from house to house warning residents to evacuate. A perimeter of 15 square miles was closed off to contain the fire.

Two smaller fires were contained by early evening.

Flames licked the shoulders of one road as cars and pickups rolled by, some towing livestock trailers.

"It's really terrifying," said Pamela Taylor of nearby Louisville, who was trying to find out whether her horse, Zorro, had been moved from a boarding stable in an evacuation area.

An emergency animal shelter at the Boulder County Fairgrounds in nearby Longmont was full, and an overflow shelter was set up at a ranch.

West said the fire apparently started when winds knocked down a power line.

"This is what scares everybody to death, these high winds," said John Stobbelaar, a retired captain with the Mountain View Fire District who went to one of the fires after he was called up on standby. Two smaller fires were contained by early Wednesday evening.

Winds at the scene ranged from 35 to 55 mph, said Kyle Fredin, a National Weather Service spokesman.

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