BOULDER, Colo. (AP) — Investigators believe a wildfire that forced the evacuation of nearly 20,000 people in northern Colorado last month was started by a very small campfire near a hiking trail, authorities said Thursday.

However, investigators have exhausted all their leads and have not been able to identify a person responsible for starting the wildfire that broke out March 26 on the outskirts of Boulder, the Boulder County Sheriff’s Office said. The investigation has been stopped but will be reopened if there are any substantial new leads, it said.

The remains of the campfire were found a few feet off a trail on public open space property just inside the city limits of Boulder, south of the National Center for Atmospheric Research, the federal facility the fire was named after, the sheriff’s office said. It is not known how old the campfire was, sheriff’s spokesperson Carrie Haverfield said.

The 190-acre wildfire burned to within 1,000 yards (914 meters) of homes on the west end of the college town, near the area where more than 1,000 homes were destroyed by a wildfire pushed by strong winds in late December.

The cause of December’s fire is still being investigated by the sheriff’s office.