Witnesses reported that after takeoff, the pilot flew by a cabin at approximately 200 ft, signaled by rocking the aircraft's wings, then began a gradual climb. The climb increased in steepness until one witness could see the entire top of the aircraft. It then nosed over, made 1 1/2 to 2 1/2 turns and crashed. An investigation revealed no specific preimpact malfunction or failure. However, an investigation revealed that the aircraft had received 3 major modifications, 2 of which were under Supplemental Type Certificates (STC's). A modified elevator bellcrank had been installed, but there was no floor or protective covering installed over the bellcrank. Only a canvas barrier protected the bellcrank from items placed in the baggage compartment. The edges of the canvas did not extend to the sides, top and bottom of the fuselage, but instead, there was a 2 to 3 inch cap, laced with string to hold the canvas secure. A whisk broom was found in the proximity of the elevator bellcrank.