On March 12, 2016, about 1330 central standard time, an experimental amateur-built Brockman Challenger II, sustained substantial damage when it impacted trees and landed in a marsh after a reported loss of engine power near Neshkoro, Wisconsin. The non-certificated pilot, the sole occupant, was seriously injured. The airplane was registered to a private individual and operated by the pilot under the provisions of the 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. The airplane departed about 1315 from a private grass airstrip located about 3.5 miles north of Neshkoro, Wisconsin. The pilot reported that he was flying for about 15 – 20 minutes and then returned to the grass airstrip. He did a low pass over the runway at 50 ft above ground level (agl) with the power at idle. About 1/2 way down the runway, he applied full throttle but the engine did not respond. He entered a shallow left turn and slowed the airplane to stall speed before hitting 2 trees about 8 ft off the ground. The airplane hit the ground and spun around 180 degrees. The owner of the grass airstrip reported that he was flying his airplane and that he was about 1/4 mile behind the accident airplane approaching the airstrip from the north. He reported that he saw the accident airplane at 150 – 200 ft agl when it turned a left crosswind and then a left downwind. He saw the accident airplane stall into the trees. He stated that the airplane impacted the trees about 15 – 20 degrees nose down with the wings level. The postaccident examination of the airplane revealed that the wreckage was located in a wooded marsh about 470 ft east of the airstrip's midfield point. (There were open fields to the south and west of the airstrip) The left wing was separated from the fuselage and fragmented. The composite left fuel tank separated from the wing but it was intact and fuel was present in the tank. The right wing remained attached to the fuselage but exhibited aft crushing. The fuel tank was intact and contained fuel. The cockpit exhibited extensive crushing and fragmentation. The empennage remained intact. The flight controls exhibited continuity from the flight controls to the control surfaces. The throttle was checked and it operated. The composite 3-bladed propeller remained attached to the reduction pulley and all 3 blades exhibited minimal damage. The leading edge of one blade was partially de-bonded, but there were no leading edge gouges or nicks. The engine was removed from the airframe and shipped to a facility for an engine examination. The examination of the engine was conducted with National Transportation Safety Board oversight. The examination revealed that the spark plugs appeared to be new with the proper gap set. The engine was rotated and compression was good on both cylinders, and drive train continuity was confirmed. The exhaust system was removed and it was in good condition. The pistons were normal in appearance with no sign of scoring or heat distress. Fuel was found in both carburetor bowls. The fuel jets were the correct size and the carburetors were set correctly. The engine was placed on an engine stand and the engine started. It operated smoothly at idle and at full power. The accident pilot was not certificated. He did not hold a current student pilot certificate and he did not qualify as a sport pilot because he had been denied a medical certificate. He had not taken the practical test for either a sport pilot's or private pilot's certificate.