Investigators said the Fraiser Fire started next to Idaho Highway 17 between Garden Valley and milepost 4.9. A discarded cigarette likely started the blaze.
The fire was reported at 2:21 p.m. Sunday. One hundred thirty firefighters and equipment from several agencies battled the blaze, which burned within half a mile of 40 homes in a nearby subdivision.
The fire was declared fully contained at 6 p.m. Tuesday.
Officials are urging anyone with information about the fire or the person responsible to contact investigators.
Investigators can be reached at 208-373-4195.