A cigarette caused the Hi Meadow fire that burned 11,000 acres in Colorado in June, officials say.
But because the cigarette was charred by the fire, investigators have no chance of identifying the smoker who caused the $18.5 million fire, said Sherrie Muldoon, Park County Sheriff Fred Wegener's assistant.
The case will be closed unless someone comes forward with more information, she said.
"We know it was a cigarette, but we don't know who put it there," Wegener said in a news release.
Muldoon said Park County has exhausted its investigative resources in the Hi Meadow fire. The Colorado Bureau of Investigation also has analyzed the burned cigarette, but CBI experts weren't able to come up with additional information.
"We can't even tell what brand it was," Muldoon said. "And there's no possibility of DNA testing because the cigarette was so burned."
Investigators declined to disclose where the cigarette was found, how it was identified as the cause of the Hi Meadow fire or who they believe might have been in the area.
"We can assume it was lit, but it's an unpopulated area," she said. "And reports that juveniles were seen fleeing the area are wrong."
The Bobcat fire, which raged through 10,600 acres and consumed 16 homes and buildings in Larimer County at the same time the Hi Meadow blaze destroyed 51 homes and buildings, was also human-caused, officials say.
A suspect was identified, but no witnesses were found and the man was not charged.
The two fires, which raged June 12-16, caused more than $18.5 million damage and cost the state nearly $5.1 million for firefighting.