BOSTON — Investigators trying to determine the cause of this crash liken it to putting together pieces of a jigsaw puzzle - mangled pieces strewn 120 feet deep on the ocean floor.
As fragments TWA Flight 800 are hauled from the Atlantic's depths, they are being put back together in a hangar in Calverton, N.Y.
With a little more than half of the plane attached to the scaffold frame, investigators still cannot say conclusively what caused the Boeing 747 jumbo jet to fall from the sky on July 17. It may take "some time" to determine the cause, according to Robert Francis, National Transportation Safety Board vice chairman.
The two-track investigation, led by the National Transportation Safety Board and the FBI, has from the beginning considered three possibilities: a bomb was placed on the plane, a missile struck the plane, or the plane experienced a mechanical malfunction.
So far, "no one says there is evidence to lean one way or another," Mr. Francis said over the weekend. Investigators have always said the bomb theory was the most likely, but they have not retrieved conclusive forensic evidence. A Boeing engineer, though, says the missile and mechanical-failure theories are still highly possible.
Early on, officials thought a bomb may have been placed in the cockpit or first-class section, because that part of the plane was severed from the rest. But all those portions of the plane have been recovered and show no signs of an explosion or a fire.
Investigators are now interested in the plane's midsection, where they believe a huge explosion occurred. In particular, they are looking at rows 17 through 28, which sit above the center fuel tank. Portions of the tank that have been recovered showed "a lot of burning," Mr. Francis said. More seats in this area of the plane were burned than elsewhere, he added.
As the recovery and investigation head into a second month, finding traces of explosives becomes more remote. But other methods exist for tracing the downing to a bomb, said James Kallstrom, the FBI investigator in charge of this case.
The FBI's focus has turned to an unidentified man who may have preboarded in New York but who was reportedly escorted off the plane for lack of identification. The FBI is also interviewing all those who flew from Athens to New York on the 747's last flight.
What the Evidence Reveals to Date
In the month since TWA Flight 800 plummeted into the waters off Long Island, N.Y., killing the 230 people aboard, investigators have not been able to determine the cause of the crash. As the process of recovering and examining the evidence continues, they are still considering three theories: a bomb, a missile, or a mechanical malfunction. Here is what's been concluded so far:
*Radar shows the plane split in two. As the cockpit and first-class section fell to the sea, the back end flew on for some 24 seconds.
*The "black boxes" show nothing unusual in the plane's mechanical functions. The voice recorder picked up no discussion of trouble in the cockpit, but a loud noise was recorded a split second before the recorder stopped.
*All four engines have been recovered intact. They show no signs of explosion or mechanical failure.
*The cockpit, front cargo hold, and front galley section have been recovered and show no signs of explosion or mechanical failure.
r*The investigation is focusing on the center of the plane, from rows 17 through 28. Pieces of this section that have been recovered, including passenger seats, show signs of intensive burning.
*Parts of the center fuel tank have been recovered. Two of six sections recovered show severe burning. This tank was carrying fewer than 100 gallons of fuel, and officials have theorized that either a leak in this tank or vapors could have ignited.
*Pieces of wreckage have tested positive for traces of explosives by a detection machine in use at investigators' Long Island headquarters. But the FBI laboratory in Washington has not corroborated those findings.
The Recovery Process
About 50 percent of the plane has been lifted from the Atlantic. Bodies of 204 crash victims have been found.
The plane is being reconstructed in a hanger in Calverton, on Long Island.
The NTSB has 30 investigators working around the clock. The FBI has "in excess of 200" agents investigating this case. Boeing, Pratt & Whitney (engine manufacturer), TWA, and the FAA each has a handful of mechanics and engineers assisting.
The US Navy has committed three ships and 650 personnel to help in the recovery, including 120 divers who have made more than 2,500 dives.
All investigative and recovery personnel work around the clock in 12-hour shifts.