Fearing further devastation, Boulder city officials had urged roughly 9,000 residents Thursday afternoon to prepare to evacuate at a moment's notice because of conditions that were ripe for fire - low humidity and strong winds. But as night fell, federal officials calmed fears of the risk to the university city of 100,000, even as gusts surpassed 40 mph.
"I'm not aware of any real threat to the city of Boulder right now," said Jim Thomas, head of the national team managing the fire effort.
Crews contained 45 percent of the 10-square-mile fire, though the line wasn't continuous. Thomas declined to speculate when it would be fully contained.
The blaze, dubbed the Fourmile Canyon fire, erupted Monday and quickly left smoking rubble where mountain neighborhoods, filled with million-dollar homes and scenic mountain cabins, once stood. Slopes of charred trees created landscapes resembling a barren winter with gray ash instead of snow.
Still, no more homes were lost to the fire Thursday. Thomas said there were no major signs of crowning, when hungry flames race along the crowns of trees or shrubbery.
Westerly winds of 25 mph with higher gusts forecast for Friday could complicate the efforts of 700 firefighters plus support personnel and air tanker crews. Parts of central and northern Colorado and southern Wyoming remained under a red flag warning Friday, meaning conditions were ripe for fire. And in an abundance of caution, Thomas refused to rule out further trouble west of Boulder.
The cause of one of the fire that destroyed more homes than any other blaze in Colorado history remained under investigation. Authorities were looking at whether a vehicle crashed into a propane tank and set it off.
About 3,500 people have been out of their homes since Monday, many frustrated by a lack of lack of information about what was happening behind fire lines. Some got around roadblocks by hiking and biking in. A limited number stayed.
Across the United States, about 2.6 million acres have burned this summer, about 50 percent less than the 10-year average, according to the National Interagency Fire Center in Boise, Idaho.