BOSTON — Two months of investigation into the crash of TWA Flight 800 off Long Island have produced no specific answers, but a deepening mystery.
A terrorist bomb remains the most likely explanation for the Boeing 747's plunge into the Atlantic, according to many experts. But plane debris dredged from the ocean depths remains strangely free of the marks such an explosion would leave behind.
The result: Officials are looking a "little harder" at the possibility that a mechanical malfunction caused the plane to crash, according to a National Transportation Safety Board spokeswoman.
Federal Bureau of Investigation and NTSB investigators, however, still insist all three scenarios - a bomb, a missile, or a mechanical failure - are being considered.
About 85 percent of the wreckage has been retrieved from the ocean floor, including about 65 percent of the center fuel tank. Investigators know that tank exploded, but they don't know what caused it to explode.
The metal in and around the tank is not pitted or flayed in a manner consistent with bomb damage, and it shows no sign of missile penetration, according to Shelly Hazle, NTSB spokeswoman.
Two of the three fuel pumps attached to the center tank have been located and checked out by NASA officials. They say the pumps show no signs that they may have provided the spark necessary to ignite the fuel vapors in the nearly empty tank. The third fuel pump, called a scavenge pump and used to drain fuel out of the tank, has not yet been located.
Seven fuel probes, only one of which has been found, are the only components that introduce electricity into the fuel tank. "They essentially have 1/20th the electrical current you would need to get a big enough spark to ignite the fuel, but you never know, and we are all obviously very interested in these," says one source close to the investigation.
Boeing has conducted some tests - in cooperation with the NTSB - on 747s to determine if the temperature of the center tank reached a point warm enough to ignite. "We went back and tried to replicate what the temperature in the tank would have been," says the source close to the investigation.
Investigators reenacted Flight 800 in a desert location that had a ground temperature about the same as that at JFK Airport the night of the crash. They ran the air conditioners for a couple of hours before take-off, taxied at the same speeds, and climbed at the same rate.
"The test indicated the fuel vapor was warm enough to be ignitable if you introduced a spark," the source says. "But we have to figure out the source of ignition."
Investigators are now considering blowing up a 747 to see if that sheds any light on why the plane crashed July 17. They have not done this before. "We'd be interested in blowing the center fuel tank, then looking at how the fuselage fails and compare that to the wreckage we have from 800," Ms. Hazle says.
Investigators have been piecing recovered parts of the 747 together in a hangar in Calverton, N.Y. But efforts to recover the plane parts have slowed recently due to bad weather. The NTSB estimates it can recover what's left of the plane in one month if the weather is fair.
Most experts still say the bomb theory is the most likely, although FBI officials said Friday that the chemical residue found on two parts of the plane - the most conclusive proof they had that pointed to a bomb - may have been left there during an exercise with bomb-sniffing dogs that took place on the plane at the St. Louis airport in June.
"If it were a conventional explosive - one they're accustomed to - they probably by now would have found some pretty conclusive evidence," says Clinton Oster, professor of public affairs at Indiana University at Bloomington and co-author of the book "Why Airplanes Crash." But "maybe it's a different type of explosive than they've been looking at in the past."
The missile theory also is still alive. Some 100 eyewitnesses have told investigators they saw an unusual streak of light heading toward the plane before the explosion. But no evidence of a missile penetration or fragments have been found.
FBI assistant director James Kallstrom has all but ruled out the possibility of "friendly fire" - a missile fired by United States military personnel in the area.
Frank McGuire, an aviation consultant, says the missile theory is far-fetched. "If anyone watched the Gulf war coverage, they would have heard the noise missiles make," he says. "Has anybody reported that screeching noise?"
Dr. Oster says it is not unusual for an investigation to take this long, considering all the evidence is more than 100 feet below the ocean's surface. "I'd be really surprised if they can't figure this one out," he says. "The NTSB is very methodical, thorough - they are able to bring to bear some of the top engineering and science folks anywhere."