The acrobatic airplane was destroyed after impacting terrain during maneuvering flight near Boise, Idaho, at about 1800 Mountain time. The Airline Transport pilot and sole passenger were seriously injured. Visual conditions prevailed. A witness reported that the airplane was coming out of a loop on the downside of the maneuver when it failed to pull up in time to clear the ground. The witness stated that he saw the airplane impact the ground in a slightly nose-up attitude, cartwheel after its left wing impacted the ground, then observed a cloud of dust before the aircraft came to rest inverted. The wreckage path was 345 feet long from initial ground impact to the final airplane component.