Investigators into the explosion of TWA Flight 800 have examined theories put forward by 500 letters from the public, the head of the National Transportation Safety said at the start of a hearing into the tragedy.

James Hall said: "Some of these theories are just not possible, but of those that were, I can assure you that we had already examined most of them and we made sure we looked into the rest."

He described the week-long public hearing into last year's midair explosion of the TWA jet off the coast of Long Island as "the largest investigation of a transportation accident in the nation's history". The cause of the explosion remains a mystery.

Earlier, the US National Transportation Safety Board (NTSB) issued thousands of pages of details of its research into the explosion aboard TWA Flight 800, and has made them available on the Internet.

The crash killed all 230 people on board the flight from New York to Paris. NTSB officials hope that the hearings may help clarify the circumstances surrounding one of the most mysterious air disasters in US history.

During the five-day hearing the research teams involved in the investigation are facing questions under oath from technical experts and safety officials at the Baltimore Convention Centre.

The public - including relatives and friends of people who died in the disaster - may attend the hearings but will not participate.

On July 17, 1996, the TWA jet was climbing above 13,000 feet when it suddenly burst into flames and plunged into the Atlantic.

John Seaman from New York, whose niece died in the crash, said: "Our goal is to try and learn as much as we can and press for accountability and justice."

"If it is a mechanical problem and they begin to zero in on the cause, which it appears that they are, then we want the problem fixed. We don't want this happening to anyone else," he said.

The safety board took sole responsibility for the investigation last month when the Federal Bureau of Investigation ruled out the possibility that a criminal or terrorist act was behind the tragedy.

Theories abound

The inquiry aims to pinpoint if there could have been an electrical fault with enough energy to ignite the fuel and air fumes in the Boeing 747's central fuel tank.

However, the newly-released research suggests that this and some of the other popular theories about what might have sparked the explosion might not stand up to the evidence.

For example, tests performed by an outside laboratory found that fuel flow could generate enough static charge to cause ignition, but not in the conditions believed to exist in case of TWA 800.

Suspicion has also fallen on the scavenge fuel pump in the central tank that was not among the 96% of the wreckage recovered from the ocean floor after the crash.

"No evidence was found that the scavenge pump in the accident airplane had been powered at the time of the accident," the NTSB aircraft systems group reported, based on pump switches found in the off position.

Another possibility is that induced voltage spikes in wires going to the fuel-measuring probes in the centre tank also seemed to be ruled out as a cause of the crash due to the lack of "arcing," or damage that would have been caused by a spark.

Investigators found no evidence of a bomb or missile attack

That also goes for the theory that wires to those probes may have failed. "No evidence of arcing was found on wires or fuel probes from the B-747 fuel probes or compensators," the report said.

The report also seems to caution against a complicated theory advanced by some experts, in which the far right wing tank may have exploded first and passed that flame inboard to the centre tank through various vent tubes.

Against this background, the NTSB is indicating it is still a long way from declaring what caused the crash. Its Public Affairs Director, Peter Goelz, said the hope was to have another hearing before the end of next year that would give a probable cause.

While this uncertainty continues, a relatives' group, the Victims of Flight 800 group, is calling for the grounding of all Boeing 747 series 100 aircraft until measures are taken to eliminate the risk of an explosion.

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