The search for wreckage from Trans World Airlines Flight 800 changed its focus Tuesday after divers essentially cleared the last major debris field, and dozens of hard-hat divers instead donned scuba gear and fanned out to hunt for the last scattered fragments of the shattered jumbo jet.
Signaling the change, the Navy ship Grapple on Monday evening raised the three heavy moorings that had circumscribed the 400-by-400-yard area at the heart of the crash site, said Comdr. Gordon Hume, a Navy spokesman.
Tuesday, the Navy towed a sensitive sonar probe through the area, looking for any small bits and pieces left behind.
The sonar sweep was part of an expanded search of a 23-square-mile rectangle of the sea bottom that was expected to be finished in “several days,” Hume said. Several crash investigators estimated that the salvage operation could be wrapped up in less than 10 days.
After almost 11 weeks of a massive but inconclusive investigation, federal law enforcement officials said they still hoped to find a piece of wreckage that would demonstrate what most of them still believed: that a bomb or, less probably, a missile caused the plane’s destruction.
Their conviction that the crash was a criminal act, not an accident, was not substantially shaken by the recent disclosure that some test results showing traces of explosives on the wreckage might have come from a bomb-sniffing exercise conducted on the jet five weeks before it crashed.
The persistent confidence that the crash was most likely sparked by an explosive device, according to several law enforcement officials and federal aviation investigators, comes in part because that explanation mostly neatly fits all the available evidence.
The evidence includes the instant stoppage of recorders tracking cockpit noise and flight data at a point when all operations seemed normal, the potency and tight focus of the initial blast and the similarity of the destruction of the plane to that seen in past bombings.
At the same time, no one has found any clear evidence of mechanical fault, though investigators are studying several possible scenarios.
But confidence that a bomb or missile brought down the plane does not of itself bring “critical mass,” the phrase used repeatedly by James K. Kallstrom, the senior law enforcement official in the inquiry. That is what he says he needs for the FBI to take over the inquiry and publicly seek a perpetrator.
And the most important piece of physical evidence would be a piece of metal that bears the distinctive physical damage that would be caused by a bomb.
Even with some 95 percent of the wreckage already on shore, law enforcement investigators have yet to find a microscopic pitting and cracking in metal shards that is one of the only indisputable signatures of a blast from high explosives.
In the meantime, investigators are pressing ahead with tests as each new truckload of wreckage arrives at an airfield in Calverton, N.Y., where the fuselage and innards of the plane are being reconstructed in several hangars.
The continued focus on forensic signs of a bomb or missile has been driven by the continued lack of evidence of a mechanical failure that could have sparked the jet’s center fuel tank to explode on its own, investigators say.
And while it remains an object of study, no one has been able to show how such a blast, on its own, could be forceful enough to cut all power instantly and split the plane in two.
Only one of the seven fuel-measuring probes from the tank has been found, and tests in Washington last week failed to point to a flaw that might have caused a spark capable of igniting jet fuel vapor, said Shelly Hazle, a spokeswoman for the National Transportation Safety Board.
Investigators have been holding out hope that divers might find some of the six missing fuel probes and the single fuel pump from the tank.