Boulder County Sheriff’s Office investigators believe that the cause of the 74-acre Sunshine Fire west of Boulder was a campfire on Boulder open space.

Investigators say that evidence at the fire’s point of origin near the Centennial Trailhead indicated the blaze was started by a campfire in what appeared to be a transient camp, according to a release.

According to the release, the fire was surrounded by rocks in a “hastily fashioned ad hoc campfire ring of sorts.” Investigators found moisture and kicked up dirt, which led them to believe an attempt was made to extinguish the campfire at one point in time.

There are no suspects at this time, and the fire remains under investigation.

The cost of fighting the fire has been estimated at $725,000.

Several neighbors indicated campfires and transient camps had become a frequent sight in the area.

The city of Boulder prohibits camping and campfires on all city open space land, and violations can result in a $1,000 fine and 90 days in jail. Boulder County is also currently under a fire ban for areas west of Broadway.

The two men accused of starting the Cold Springs Fire in 2016 by not properly extinguishing a campfire were also charged with arson and sentenced to work release.

The fire was first reported at about 1:40 a.m. Sunday by a man who said he saw flames near Sunshine Canyon and Timber Lane.

The fire prompted mandatory evacuations for 426 homes and pre-evacuation notices for another 836 homes before crews were able to reach full containment on the fire Monday afternoon. No structures were damaged.

The cost of fighting the fire has been estimated at $725,000.

Anyone with information in the investigation of the Sunshine Fire should contact Boulder County sheriff’s Detective Jason Shatek at 303-441-3641. Those who wish to remain anonymous may contact the Northern Colorado Crime Stoppers at 1-800-222-TIPS or nococrimestoppers.com/

Mitchell Byars: 303-473-1329, byarsm@dailycamera.com or twitter.com/mitchellbyars.