On April 17, 2004, about 1200 mountain daylight time, a Keller Prospector FK1 airplane, N5XK, sustained substantial damage after colliding with forested terrain near Lost Trail Pass, Gibbonsville, Idaho. The airplane is owned by the pilot and was being operated as a visual flight rules (VFR) cross-country flight under the provisions of 14 CFR Part 91, when the accident occurred. The pilot, and pilot rated passenger, both sustained serious injuries in the accident and were transported to area hospitals. Visual meteorological conditions prevailed; however, areas of low ceilings and reduced visibility were reported in the area of the accident. The flight originated from Stevensville, Montana, approximately 1 hour and 30 minutes prior to the accident. The pilot's planned destination was Jackpot, Nevada (06U). During a telephone conversation with the National Transportation Safety Board IIC, on April 20, the pilot rated passenger reported that the accident airplane was one of three aircraft that was flying from Montana to Nevada. The pilot rated passenger reported the weather conditions along the route of flight began to deteriorate, and the pilot-in-command of the accident airplane elected to return to Stevensville. The passenger reported that shortly after making the decision to return, the airplane's engine began to lose power. After loosing power, the airplane stalled and collided with terrain. The pilot rated passenger reported the loss of power was the result of carburetor ice. In a written statement dated May 15, 2004, the pilot reported that approximately 10 miles south of Lost Trail Pass he "decided to turn back due to deteriorating weather ahead." The pilot stated that during the turn he lost track of the two other aircraft that made up the flight of three traveling together to Jackpot. The pilot stated he was "trying to locate [the] other two A/C [aircraft] to the exclusion of the engine instruments". The pilot stated that as the airplane reached the summit he added power "as I was too low." Concurrent with the increase in power, the engine quit. The pilot maneuvered the airplane in an effort to avoid rising terrain, however, during the maneuver the airplane collided with multiple trees eventually impacting terrain in an inverted position. In the Recommendations section "How Could This Accident Have Been Prevented" of the NTSB Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), submitted on May 14, 2004, the pilot reported, in part, that the use of carburetor heat may have prevented the accident.