On March 6, 2004, about 0915 eastern standard time, an unregistered Quad City Aircraft Corp., Challenger II aircraft, operated by a private individual as a Title 14 CFR Part 91 personal flight, crashed in Labelle, Florida. Visual meteorological conditions prevailed, and no flight plan was filed. The pilot and one passenger received fatal injuries, and the aircraft was destroyed. The flight originated in Labelle, Florida, the same day, about 0900. A witness stated that he was flying his ultralight aircraft in close proximity to the accident aircraft and he observed the accident aircraft enter about a 45-degree dive from an altitude of about 500 feet. He further stated that he was behind the accident aircraft and he observed it as it dove toward the ground. From his position, he said he saw the attitude of the aircraft change abruptly, as if to recover from the dive at an altitude of about 50 feet above the ground. As the nose pitched up, the witness stated that he saw the right wing separate, and the aircraft then impacted the ground. He also said that he had observed the accident aircraft during one other such dive and recovery maneuver, immediately preceding the maneuver during which the accident occurred. Postcrash examination of the right wing spar attach bracket by an aircraft builder/ultralight mechanic, who observed the accident, revealed that the bracket had separated consistent with overstress. Records did not indicate the existence of an exemption to operate the aircraft as an ultralight.