Washington, Aug. 22 (Bloomberg) -- U.S. safety officials began a final review of Trans World Airlines Inc.'s fatal Flight 800 crash without citing a specific cause, though they again ruled out theories that the 1996 incident was the result of a criminal act. The National Transportation Safety Board ``unequivocally'' found no evidence that the crash, which killed all 230 people onboard, was caused by a criminal act, NTSB Chairman Jim Hall said. Today's meeting marks the end of the longest and costliest accident investigation in U.S. history, stretching to 15,000 pages of written evidence and comments, he said. ``Those who consistently distort the record and persist in making unfounded charges of a cover-up'' were doing ``a disservice to us all but most especially to you, the TWA 800 families, who have suffered so much in this tragedy,'' Hall said at the meeting at his agency's Washington headquarters. The Boeing Co. 747 aircraft exploded off New York's Long Island on July 17, 1996.

The safety board has focused on a search for what ignited vapors in a near-empty center fuel tank of the 25- year-old plane. Other observers, such as former Navy pilot James Donaldson, have said the plane was shot down by a missile. Today's testimony continued to focus on the circumstances of the explosion without pinpointing a specific cause. The two-day meeting will assess progress on NTSB recommendations in 1996 and 1998 to alter fuel-tank design and make operating changes to improve safety, Hall said. ``In the early months of the investigation, it became clear that an explosion of flammable vapors in the aircraft's center wing tank initiated the breakup and subsequent crash of Flight 800,'' he said. Flammable Vapors The NTSB said it knows that fuel vapors can explode, though it isn't sure what caused the spark that triggered the Flight 800 explosion. ``The fuel/air vapor in the center wing tank was flammable at the time of the accident and a fuel/air explosion with Jet A fuel was more than capable of generating the pressure needed to break apart the center wing tank and destroy the airplane,'' said investigator Bernard Loeb.

The NTSB ruled out all ignition sources other than a short circuit in electrical wiring outside the wing tank that ``somehow transferred excess voltage'' to a fuel-quantity indicator inside the tank, Loeb said. ``We cannot be certain that this (short circuit) in fact occurred,'' Loeb said. ``But of all the ignition scenarios that we considered this scenario is the most likely.'' Conditions inside the fuel tanks of U.S. airliners still can be conducive to explosions, especially when the plane's fuselage is heated by air-conditioning packs used to cool passengers before takeoff, the agency said. Fuel-Tank Tests The NTSB evaluated the fuel tanks in tests over almost four years to see what conditions could cause the vapors to explode. In tests on a leased 747, wing tank temperatures were measured in 200 different locations. Temperatures in the vapor space above 60 gallons of fuel left in the 12,490-gallon tank reached at least 120 degrees Fahrenheit. That was above the 100- degree ignition temperature at the 13,800-foot altitude at which the plane exploded, NTSB investigator Joseph Kolly said. The tests also showed that jet fuel can be ignited with less energy than the static electricity generated when a person crosses a room and touches a doorknob, he said. While the U.S. Federal Aviation Administration has proposed steps to reduce heat transfer to the fuel tank, the NTSB believes more should be done to reduce the chance of jet-fuel explosions, Loeb said.

The NTSB is concerned that the FAA hasn't completed new rules that would require airlines to upgrade aging wiring systems, after deteriorated wiring was found in the TWA wreckage, he said. The most effective way to prevent fuel-tank explosions is to eliminate the flammable vapors while minimizing ignition sources, Loeb said. The NTSB's draft final report, which is being presented during the meeting, doesn't include any new recommendations on fuel-tank flammability. A technique in which inert gas is pumped into tanks before takeoff is ``the most promising method for dramatically reducing fuel-tank flammability,'' Loeb said. The NTSB first recommended in 1996 that the FAA investigate the technique, known as inerting. After testing the technique in the past year, the FAA earlier this year said it has commercial promise because the cost has been reduced by as much as 75 percent to about $800 million. Traces of Explosives Loeb said traces of explosives were found on the plane, though he ruled out a bomb as a cause.

``We don't know exactly how the explosive residues got there, but we do know from physical evidence ... that the residues were not the result of the detonation of a bomb,'' he said. The NTSB believes the ``streak of light'' seen by 258 crash witnesses was burning fuel from the damaged airplane and not a bomb or missile, Loeb said. The Federal Bureau of Investigation's concurrent investigation also concluded that the crash wasn't the result of a bomb, missile or other criminal act. The safety board spent $35 million on the recovery effort, finding 95 percent of the aircraft's fuselage and the remains of all the victims. The NTSB also leased a Boeing 747 to study the center wing tank's structure and evaluated the explosive properties of the Jet A fuel used.

The meeting is being carried via the Internet at the agency's Web site, http://www.ntsb.gov. Airlines and other U.S. federal agencies have done their own crash investigations. The FAA has concentrated on removing ignition sources, issuing 37 orders to airlines to revise airline wiring and fuel- tank maintenance. Boeing, the world's largest planemaker, included issuing 48 service bulletins that covered modification and inspection of its fleet. The Air Transport Association airline trade group, along with the Aerospace Industries Association the represents planemakers, inspected almost 1,000 aircraft and concluded that aircraft fuel tanks were in sound condition and weren't an explosion risk. Shares of St. Louis-based TWA, which had traded as high as 23 3/4 in the months before the 1996 accident, rose 1/16 to 2 1/8.