The final US Air Safety Agency's report on the TWA flight 800 tragedy said a design flaw in the Boeing 747-100's central fuel tank caused the mid-air explosion that killed 230 people in 1996.
"The probable cause of the TWA flight 800 accident was an explosion of the centre wing fuel tank resulting from ignition of the flammable fuel/air mixture in the tank," the National Transportation Safety Board (NTSB) concluded in its report, released late on Wednesday.
The Boeing 747-100 was on Flight 800 from New York to Paris when it exploded in mid-air on July 17th, 1996, just 12 minutes after take-off from John F. Kennedy airport. The blast and the crash killed all 212 passengers and 18 crew members aboard.
The report also highlighted the contributing factor to the explosion as "the design and certification of the Boeing 747 with heat sources located beneath the centre wing tank with no means to reduce the heat transferred into the centre wing tank or to render the fuel vapours in the tank non-flammable".
The heat sources referred to by the NTSB are the air conditioner generators located directly beneath the central fuel tank, which as well as generating cool air for the passenger cabin, release significant amounts of heat.
The inquiry said the generators were a key factor in heating the mix of air and kerosene fumes of the TWA aircraft on the day of the accident.
The aircraft was parked on the tarmac with its air conditioners running for several hours before the flight, which had a two-hour delay before take-off. Investigators' tests showed that the temperature inside the central tank may have reached 52 degrees. The combustion threshold for kerosene is 35 degrees.
The source of the ignition for the explosion could not be determined with certainty but, of all those evaluated by investigators, the most likely was a short circuit outside the centre wing tank that sent explosive voltage inside through the electrical wiring of the fuel quantity indication system.
After the NTSB report was released, the Boeing company issued a statement undertaking to enhance the safety of all its aircraft, especially the Boeing 747100. The company stressed that after the accident it had adopted measures "to reduce the flammability of fuel vapour" and "to further reduce the possibility of ignition sources inside airplane fuel tanks".
Boeing said it had sent 48 service bulletins recommending improvements in fuel-delivery systems to airlines using their aircrafts. They said they had spent more than $30 million researching the causes of the TWA 800 crash. The NTSB investigation cost some $35 million.
Before the official release of the NTSB report, an independent panel of experts on Monday criticised the investigation results, and maintained its theory that the aircraft was brought down by a missile.
According to the Flight 800 Independent Research Organisation (FIRO), the official investigators concealed crucial pieces of information from the final report.
Many former investigators, military experts, and airline pilots continue to insist the Boeing was shot down by a missile. "Thirteen witnesses have seen an object strike the plane," the FIRO President, Mr Tom Stalcup, said.