THE leading theory as to what might have caused TWA 800 to plunge into the ocean off Long Island almost a month ago - the detonation of a bomb in the front cargo hold - has been thrown into serious doubt.
An examination of all four containers in the area has shown no evidence of an incendiary blast.
The last of the large metal boxes into which passenger bags had been stowed was retrieved from the sea on Sunday and appeared to be essentially intact, if somewhat battered.
Mr James Kallstrom, the FBI agent in charge of the investigation, conceded that all four containers "are basically unremarkable".
Investigators were drawn to the theory of a bomb placed inside one of the containers in part because of a finding that the front section of the aircraft had become detached in the accident and had fallen to the ocean first.
They were also attracted by parallels with the downing of Pan Am Flight 103 over Lockerbie, Scotland, in 1988, which was traced to a bomb in the front hold.
The mystery surrounding the loss of TWA 800 therefore appears, if anything, to be deepening.
Investigators are far from giving up on the idea that a bomb caused the explosion.
However, they remain unable to rule out two other possibilities: that the plane was brought down by a missile or by a massive mechanical failure, such as a catastrophic deployment of the engine reverse thrusters in midflight.
"We have these three theories and they're all on the board," Mr Kallstrom admitted. "Until we know exactly what happened and which theory proves to be the case, they'll remain there."
But investigators appear still to favour criminal sabotage of some kind.
The investigators said that navy teams had retrieved the bodies of 198 of the 230 victims and about half of the aircraft's wreckage.
An Israeli newspaper, Maariv, reported yesterday an FBI team is hoping to interview a Lebanese man, in detention in Israel who was injured when a bomb he was making exploded prematurely in April.
Flaws in security measures at John F. Kennedy Airport, meanwhile, are also coming under renewed scrutiny. These were graphically demonstrated by the findings of a local police officer investigating security arrangements one week after the crash.
With a limited access security badge issued to ticket clerks of a commuter airline, the officer was able to penetrate any part of the airport he chose, including baggage sorting areas and parked aircraft.
"It was incredible," the officer reported, attesting about the access he gained to the aircraft themselves. "I suppose if I wanted to I could have sat down in the pilot's seat and driven away with the thing".
Turning their attention away from the front cargo hold, investigators are now questioning whether a bomb could have been placed in a food trolley or the carry on baggage of a passenger that might have been stowed under a seat or in an overhead baggage compartment.
Interest is also being focused on the so called centre wing fuel tank, a large tank area between the wings that was only partly filled when the plane took off.
Parts of the tank already retrieved show signs of intense fire damage. If the fuel that was in the tank became warm enough to become vapour and something ignited it, it could have led to an explosion.