(CNN) -- One of the most destructive fires in Colorado history was 100% contained Thursday after burning 16,000 acres in nine days, a spokesman for the county said.
Officials made the declaration after extinguishing small areas of smoke that kept popping up in one small corner of the Black Forest Fire, El Paso County spokesman Dave Rose said.
Sheriff Terry Maketa allowed residents back in Thursday night.
The 16,000-acre Black Forest Fire killed two people and destroyed more than 500 structures, and it prompted thousands of people to flee.
"This is the worst fire in Colorado history in terms of damage," Rose told CNN.
Investigators are still trying to figure out what caused the fire. Investigators scouring a 24-square-foot area where the fire is thought to have begun on June 11 have "all but ruled out natural causes," Maketa said.
"I can't really go any further on that, but I can say we are pretty confident it was not, for instance, a lightning strike," Maketa said.
The sheriff said earlier this week the fire was being treated as a crime scene, meaning in part that investigators were preserving every piece of evidence that they could. But he said he wasn't ready to say whether a crime was committed.
The fire rapidly ravaged woods and neighborhoods in a mostly rural area northeast of Colorado Springs last week, and firefighters struggled to keep up in the early days, thanks to little rain and blustery winds
But more favorable conditions helped firefighters raise containment from 5% Friday to 65% Sunday.