With the most promising explanations for the crash of TWA Flight 800 becoming increasingly difficult to prove, the investigation has turned into a scientific exploration of all conceivable theories, including such remote possibilities as the jet being destroyed by debris falling from space or by natural gas rising from the ocean.
The widening of the search reflects investigators' fears that they will never be able to say precisely what ignited the explosion. Instead, they expect they will have to convince the public that they have ruled out a variety of possibilities _ anything from bombs and meteors to faulty wiring _ and have narrowed the possible causes to one or two that they can address.
Aviation investigators are pressing hard to show how the explosion in the plane's center fuel tank could have resulted from a mechanical malfunction and not some external force, criminal or natural. For months they have focused on the possibility that a spark could have ignited the fumes in the Boeing 747's nearly empty fuel tank.
Working in a laboratory at the Wright-Patterson Air Force Base in Ohio, engineers have tried in the past two weeks to reconstruct the conditions within the tank, but they have not been able to generate a spark strong enough to cause a blast. Those tests are expected to continue for several weeks.
Since the morning after the July 17 crash off Long Island, N.Y., that killed 230 people, officials of the National Transportation Safety Board and the FBI have listed three possible causes: a bomb, a missile or a mechanical malfunction. But since no clear evidence of any of those three has been found, the possibilities are growing to include some that investigators consider far-fetched.
They are working with scientists from military and intelligence agencies to determine whether it was possible something falling from space hit the plane _ a tiny meteor, or perhaps a piece of broken satellite the size of a BB.
And when a scientist at the Sandia National Laboratories in New Mexico recently wrote to the board that he suspected a bubble of natural gas rose out of the ocean, enveloped the plane and exploded, the agency dispatched a meteorologist who interviewed the scientist, then returned to Washington with some documents to study.
"We're looking at anything that has any degree of probability until we have a reason not to," said Bernard Loeb, director of aviation safety for the board.
The investigation has traveled well beyond the ocean, where scallop boats are still trawling for any remaining pieces of wreckage, and beyond a hangar in Calverton, N.Y., where a 90-foot section of the plane is being reconstructed. Scientists and engineers at more than a half-dozen government-financed labs have become part of the search, studying everything from how metal changes when hit by a missile to whether a computer model of an explosion in the plane's center fuel tank can be developed.
Safety investigators are working with British aviation investigators and have consulted with their Russian counterparts to try to understand whether the final loud sound on the cockpit voice recorder was made by a fuel-air explosion or by a more rapid detonation, indicating a bomb or missile.
"I think it's entirely prudent to keep all things up on the board _ even space junk and meteors _ plus all the bomb and missile and mechanical theories," said James Kallstrom, head of the FBI's New York office.
The only theory that officials have ruled out, emphatically so, is the accusation renewed last week that a Navy missile shot down the plane and that the federal government was involved in a cover-up. Officials said that Pierre Salinger, the former television correspondent and White House press secretary who has become an advocate of the missile theory, has relied on a misinterpretation of radar information and unsubstantiated conjecture to support his conclusions.
Now that more than 90 percent of the plane has been recovered and no evidence of a bomb or missile has been found, the investigation is increasingly focused on a mechanical malfunction.
The investigators announced in December that the 12,890-gallon center fuel tank was primed for an explosion because air-conditioning packs below it had heated the small amount of fuel, producing dangerous vapors. Board officials then recommended ways of keeping fuel tanks below that temperature.
Explosives experts have advised the board that searching for proof of the exact cause may be fruitless, since the blast probably destroyed conclusive evidence. "It is not unusual, in a fire-related accident, for us not to identify a point of ignition," Loeb said.
With the public still focused on the crash and the possibility of a criminal act, the government is in the midst of an intense endeavor to prove which ignition sources were possible and which could be ruled out. The scientific effort, coupled with other work on the crash, is expected to push the cost of the investigation by fall to $27-million, about 60 percent of the transportation board's annual budget.
A large part of the scientific effort being directed by the safety board is to prove what aviation investigators already think: that an explosion of fuel and air in the tank, generated by a mechanical failure within the tank, created enough force to break up the plane and cause other damage seen in the wreckage.
In an explosives lab at the California Institute of Technology, scientists are trying to determine: What concentration of fuel vapor would have been necessary to cause the explosion? What pressure would be created by the explosion? And what energy is needed to cause the explosion?
Once they establish scientific proof that the explosion could have taken place, the next step is to show that the force from the explosion could cause the damage to the plane, which has been cataloged by engineers and technicians back in the hangar on Long Island.
Using data from the original engineering plans of the 25-year-old plane and calculations based on an analysis of the damage, engineers have tried to understand precisely how the plane came apart.
Thomas Zacharia, a scientist at the Oak Ridge National Laboratory in Tennessee whose field is computational analysis, said investigators have developed a detailed understanding of how the pressure wave from the explosion traveled "and how it caused the airplane to break up and crash."
NTSB investigators said they think the explosion in the tank blew forward some of the beam structure of the tank, causing the fuselage tears that split the plane in two.
Getting a clearer picture of how the explosion worked would require a computer model of that portion of the plane, but officials at the NTSB and at Boeing say they are having difficulty developing a model that will replay an explosion. Given the fuel tank's complex internal structure of baffles and beams, Loeb said, it is difficult to discern how the force of an explosion "moves around corners."
Understanding the effect of an explosion, however, does not reveal precisely how it starts. For months, safety investigators have focused on the possibility that a spray of fuel from a leaking fuel line within the tank could cause a static charge on a piece of metal, leading to a spark. Working at the Naval Research Laboratory in Washington, safety board engineers found that spraying fuel on pieces of the metal used in a fuel tank could create an electrical charge on the metal.
To avoid a spark, all metal pieces on an airplane are supposed to be tied or bonded to another piece of metal, making the plane one circuit _ a design that also allows aircraft to survive a lightning strike. But safety board investigators have found couplings and clamps that were unbonded in other airplanes. They found similar conditions on fuel tank parts pulled from the wreckage of Flight 800, although it was impossible to determine what their condition had been before the crash. In addition, the fuel flowing through the tank at the time of the crash did not contain an anti-static agent used in other countries.