The shift in focus, confirmed anonymously this week by officials close to the inquiry, once more puts the aircraft's manufacturer, Boeing, under the spotlight. And it is certain to rekindle concerns that hundreds of jumbo jets, carrying thousands of passengers daily, may not be safe.
More than two months after the tragedy, which claimed the lives of 230 people, the formal line remains the same - that three possible scenarios are under parallel and equal scrutiny: the plane was downed by a bomb, a missile struck it, or something mechanical went badly wrong.
No new evidence has been recovered pointing to mechanical failure. The theory has returned to the fore, however, because while officials - particularly agents of the FBI - have privately favoured the bomb theory, convincing proof of a blast has still to be discovered.
"We are getting absolutely nothing as it relates to a bomb," one source told the Washington Post. "Things seem to be pointing toward mechanical malfunction and that is where investigators are concentrating now".
A rift appears to have opened between the two halves of the investigating team.
While the FBI has made little secret of its belief that the plane was downed by a criminal act, officials of the National Transportation Safety Board (NTSB) have not been convinced by the arguments.
An apparent breakthrough over proving the bomb theory several weeks ago - the discovery of traces of explosive material inside the fuselage near the right wing - is now the object of scepticism.
Officials note that such residues could have arrived in the plane months or years before the crash, for instance in transporting military personnel during the 1991 Gulf war.
And divers scouring the ocean floor for remaining wreckage have not turned up any parts showing the tell-tale signs of an incendiary explosion, such as metal pitted or dramatically misshapen by a blast.
Critical to the inquiry now is the role that may have been played by the central fuel tank of the airliner. The tank, with a capacity of 12,800 gallons, had only 50 to 100 gallons in it at the time of the plane's take- off. NTSB officials suspect that an electrical spark may have ignited fuel vapours in the tank, causing it to erupt.
Boeing has conducted its own simulations and said that an ignition in the tank could not have produced a blast strong enough to cripple the plane as instantaneously as suggested by data from the voice recorders. The NTSB intends conducting its own experiments to test this.
"If you get a fuel-air explosion in that tank, how does it vent itself?" one investigator was quoted as saying by the New York Times. "That's what we have to come to grips with."
The NTSB is said to be considering acquiring a scrapped 747 airliner to set up an explosion in the central fuel tank. Thus investigators might be able to determine the possible extent of damage from such an occurrence.
How a spark could have occurred in the tank is not certain. Possible causes could include electrically driven fuel monitors in the tank.
Normally they do not carry sufficient current to cause such a spark. Also under scrutiny are the various pumps attached to the tank's exterior. One of these pumps has still not been retrieved.
While the prospect grows that the cause of the accident may never conclusively be determined, investigators are under heavy pressure to make headway. There are big political stakes if sabotage is proved and high financial stakes for TWA, the air industry as a whole and the insurance industry if mechanical malfunction is found to be the cause.
The inclination of investigators hitherto to favour the bomb theory - few have given credence to the missile scenario - has already led Washington to introduce new measures to tighten up security at American airports.
The possibility still exists that divers could produce the one vital piece of wreckage that would end the mystery at a stroke, particularly a fragment of the plane clearly showing signs of a criminal blast.Reuse content