Post-accident investigation by the National Transportation Safety Board (NTSB) showed that the leading cause of the accident was Denver's inability to switch fuel tanks during flight. The quantity of fuel had been depleted during the plane's flight to Monterey and in several brief practice takeoffs and landings Denver performed at the airport immediately before the final flight. His newly purchased amateur-built Rutan aircraft had an unusual fuel tank selector valve handle configuration. The handle had originally been intended by the plane's designer to be between the pilot's legs. The builder instead put it behind the pilot's left shoulder. The fuel gauge was also placed behind the pilot's seat and was not visible to the person at the controls. An NTSB interview with the aircraft mechanic servicing Denver's plane revealed that he and Denver had discussed the inaccessibility of the cockpit fuel selector valve handle and its resistance to being turned.