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BOULDER, COLORADO (AP) -- Four people are missing in Colorado, where a wildfire burning in the foothills near Boulder is only 10 percent contained.

Authorities say it's destroyed at least 135 homes, making the blaze the most destructive in state history.

Spanning about 10 square miles, it's not large in terms of size. But it's in a populated area and has inflicted a lot of property damage.

With some 3,500 people forced to evacuate their homes, the Boulder County sheriff says a lot of people have lost everything, including some firefighters trying to protect the homes of others.

Fire officials were grateful for cooler temperatures and a light rain on Wednesday, but conditions are expected to worsen overnight. Winds are expected to pick up and the humidity to fall. The National Weather Service says there's a high risk of new fires spreading quickly along the Front Range.

Crews have begun building containment lines near the eastern side of the fire, while seven heavy air tankers have dropped 35,000 gallons of flame retardant.

So far, no deaths or injuries have been reported. The cause of the fire remains unknown.

(Copyright ©2010 by The Associated Press. All Rights Reserved.)