A fierce, wind-driven wildfire destroyed hundreds of homes, injured at least a half dozen people and prompted the evacuation of tens of thousands of residents in and around two towns east of the Rockies near Denver, authorities said on Thursday. The swiftly spreading prairie grass fire was believed to have been ignited by sparks from power lines and transformers toppled by high winds on Colorado's drought-parched Front Range, according to Boulder County Sheriff Joe Pelle.