in Kure Beach. All debris was accounted for as belonging to the DC-6. Another theory presented by an expert shortly after the crash theorized that an engine fire could have been the catalyst of the accident. Under this theory, one of the two engines on the right wing may have caught fire. Shrapnel from the engine may have punctured the fuselage, causing explosive decompression. Alternatively, Julian Frank, who was known to be desperately afraid of flying, may have panicked and hit the window, weakening it in such a manner it subsequently blew out. Under this theory, the pilots and passengers would have been aware of an emergency aboard, which would have allowed them to begin making preparations for an emergency landing. This theory was supported by the wide right turn the aircraft appeared to make prior to disintegrating and crashing, as well as the fact some of the passengers were found wearing life jackets. Though the bombing and engine fire theories were the most commonly held, other theories were advanced during the investigation as well. Several days after the explosion, National Airlines pilots who were members of the Airline Pilots Association sent a telegram to the Federal Aviation Administration (FAA). In the telegram, they made a claim that the routine proficiency flights performed by pilots caused unnecessary stress on the aircraft. These test flights, which pilots underwent every six months, required the pilots to put their aircraft through "violent maneuvers" which could damage the aircraft. In their final report, the Civil Aeronautics Board indicated it had investigated a variety of alternative theories, including: metal fatigue failure of the cabin leading to explosive decompression a propeller blade failing, striking, and rupturing the cabin a malfunction in the cabin pressurization system leading to structural failure a foreign object striking the plane and penetrating the cabin lightning strike fuel vapor explosion oxygen bottle explosion The CAB ruled out each of these theories during the course of their investigation. Conclusions The Civil Aeronautics Board concluded Flight 2511 was brought down by a dynamite explosion in the passenger cabin. The explosive charge was located "beneath the extreme right seat of seat row No. 7." The report also pointed out that Julian Frank was close to the explosion, though it assigned no blame to him. The explosion occurred at approximately 2:33 a.m., significantly damaging the structural integrity of the aircraft and forcing it into a wide