The Cal-Wood fire appears to burn a structure north of Boulder on Saturday night, Oct. 17, 2020. (Joseph Gruber, Special to The Colorado Sun)

Investigators are unable to determine the cause of the Cal-Wood fire, which burned more than 10,000 acres and destroyed 20 homes in Boulder County in October.

The Boulder County Sheriff’s Office said Wednesday authorities were able to narrow down a 500 foot radius of land near Jamestown where the fire started. As part of the investigation, officials reviewed community tips and interviewed landowners and those recreating in the area the day the fire started.

Officials initially said that they had ruled out natural causes because there was no lightning in the area when the blaze began. They are now walking that back.

“Because fire investigators were now able to determine a cause, concluding it is undetermined, they do not feel comfortable saying if it was human caused,” Carrie Haverfield, spokeswoman for the sheriff’s office, said in a written statement. “They were unable to attribute the cause to any one source (human or nature).”

Multiple agencies were involved in the investigation, including the Boulder County Multi-Agency Fire Investigation Team, the U.S. Forest Service, the Bureau of Alcohol Tobacco Firearms and Explosives, the Colorado Division of Fire Prevention and Control, and the National Oceanic and Atmospheric Administration.

The fire started on Oct. 17, spreading at a rate at 1,000 acres an hour. By the time it was fully contained on Nov. 14, it had burned 10,113 acres.

A separate fire nearby started on Oct. 18. The Lefthand Canyon fire, which originated near Ward, burned 460 acres before firefighters contained it on Oct. 22. The cause of that fire remains under investigation.

The cost of fighting both fires totaled $6.6 million. No one was killed or injured in either fire.

Lucy Haggard was a TRENDS Reporting Fellow from August 2020 to May 2021 with The Colorado Sun. Email: Twitter: @lucy_haggard