The Boulder County Sheriff's Office has determined that the Ridge Fire, which burned just northwest of Boulder, was human-caused.
On Monday, they tweeted that the investigation is ongoing and that the it wasn't related to either transient activity or an unattended campfire, but did not further speculate on the cause.
The wildland fire ignited Wednesday evening northwest of Boulder on a ridge between Wagonwheel Gap Rd. and N. Cedar Brook Rd.
Boulder Mountain Fire Department estimated the fire to have burned around one acre.
The department says the fire did not threaten any structures and no evacuations were ordered.