EVIDENCE OF EXPLOSIVE FOUND IN TWA DEBRIS.

Byline: Don Van Natta Jr. The New York Times

After a prolonged, confounding search of the ocean floor, investigators have finally found scientific evidence that an explosive device was detonated inside the passenger cabin of Trans World Airlines Flight 800, senior federal officials said Thursday.

Chemists at the Federal Bureau of Investigation crime laboratory in Washington have found traces of PETN, a chemical in plastic explosives, on a piece of wreckage retrieved from the jet's passenger cabin between Rows 17 and 27, according to three senior officials deeply involved in the investigation. They spoke on the condition of anonymity.

While the new finding provides evidence that the plane was destroyed by an explosive device, a senior official noted that PETN, or pentaerythritol tetranitrate, was an explosive component commonly found in many bombs and surface-to-air missiles, making it impossible, for now, to know for sure which type of explosive device destroyed the Boeing 747, killing all 230 people aboard.

Nonetheless, the discovery meets the FBI's previously stated standard for declaring that the plane was brought down by a criminal act. In loss of life, the downing of TWA Flight 800 would stand as the most serious crime in U.S. history.

For weeks, criminal investigators have said they would need positive findings of explosive residue at the Washington lab before they could conclude what most of them have believed all along - that a bomb, not an unusual mechanical malfunction - destroyed the jet shortly after it left New York City's Kennedy International Airport on the evening of July 17.

But senior investigators said Thursday that they are not ready to declare that the crash was the result of a criminal act until they can say for sure whether the explosion was caused by a bomb or a missile. A senior investigator said federal authorities could never bring any suspects to trial until they have answered that question.

He and other senior officials said they still hoped to find additional forensic evidence as salvage workers continue to retrieve more wreckage from the Atlantic. They are particularly interested in finding metal fragments showing what investigators call shock waves - physical damage left by a blast that holds signature markings demonstrating what type of device exploded.

Investigators had found preliminary indications of PETN residue in at least one earlier test. Five days after the crash, a chemical test at a makeshift lab in Calverton indicated a trace of PETN on a piece of the right wing near where it met the fuselage. But that was not confirmed by a later test at the FBI lab in Washington, officials have said.

The new, confirmed test result comes from a piece of wreckage - part of a seat, one official said - which was situated in the precise area of the passenger cabin where investigators have said the epicenter of the blast was - somewhere between Rows 17 and 27.

It is close to the area of the right wing where the earlier test showed the preliminary, positive finding of PETN residue, officials said. This is also near the spot where the plane split in two in the sky.

The test was conducted as long as two weeks ago, investigators said. But instead of announcing the finding then, FBI officials decided to take extraordinary precautions to keep the finding secret as long as possible. The information was so tightly controlled that only three or four senior investigators knew about it until several members of the investigation task force were told about it during a private briefing Wednesday.

``We haven't said it because we still don't know for sure what brought the plane down,'' a senior investigator said. ``Just having one hit is not enough. We won't say it is a bomb or missile with just one hit on the plane. It is potentially important, but it doesn't get you where you need to go.''

Another federal official said Thursday that bomb experts had several reasons to be cautious about the positive finding of explosive residue. One was they had not, as they had expected, found any other PETN residues on nearby parts of the plane.

Moreover, since PETN is usually a key component of a detonator, not a main explosive charge, they had expected to find residues of bomb chemicals as well. But, the official said, they had not yet found any evidence of it. Nor have they found the ``shock wave'' damage that typically accompanies a blast.

At a news briefing July 29, Joseph Cantamessa Jr., a special agent in charge of the FBI in New York, said that ``one positive result'' in the forensic tests would be sufficient to allow investigators to declare the TWA crash a criminal act. ``If we are not talking chemical,'' he added, ``there are probably several steps that have to be passed through.''

Other investigators echoed that view in subsequent days.

Now that investigators have that positive test result, a senior official pointed out that federal law enforcement officers never discuss physical evidence during a criminal investigation. Even if they do finally make a public announcement that a bomb, or a missile, brought down the plane, federal officers probably will not disclose the evidence that led them to the conclusion, the official said.

By saying that the FBI will not officially declare that the plane was brought down by a criminal act until they have enough evidence to present a strong legal case, investigators are raising an evidentiary threshold that could take weeks or months to meet.

Still, a senior official conceded that investigators would rather have one positive test result, rather than none, now that nearly 60 percent of the plane's wreckage has been retrieved from the ocean floor. ``Certainly, it puts in my gut more of a feeling that it was a bomb in that area,'' the official said.

After the first preliminary, positive test result, investigators said they were confident that it was only a matter of days, rather than weeks, before they would get other positive results - or even find a telling piece of wreckage from an area adjacent to the blast side. Investigators have said that piece probably would be as small as a football.

But with each subsequent negative test, at the labs in Calverton and Washington, the investigators' confidence waned. Publicly, however, they still insisted they were confident that any day they would find the evidence that would point conclusively to the cause of the crash.

At the same time, investigators said they wondered about the lack of new forensic test results that would confirm the initial result from Washington. As each day passed with no other positives found, investigators worried that they might be forced to build a circumstantial criminal case, relying totally on the one positive test result in Washington.

PETN is a chief component in the high-order plastic explosive commonly called detasheet or detcord. PETN also can be found in other explosive mixtures, like those made with Semtex.

In 1982, a bomb exploded on a Pan American flight to Honolulu. Two weeks later, another Pan Am flight, from Miami to Rio de Janeiro, was found to have a bomb on board.

The bombs were determined to have been virtually identical, and their chief component was PETN. In the Rio bomb, the PETN was molded into 4-by-10-inch strips and sewn into the lining of a suitcase.

In recent days, TWA investigators acknowledged that the lone positive forensic result in Washington has held an important psychological value for them, simply because it confirms their leading theory that the plane was destroyed by a powerful bomb, or a missile - making the crash a criminal act.

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Photo

Photo: (Color) A TWA Flight 800 engine sits in a New York s tate hangar. On Thursday, officials said an explosive was found in wreckage.

Associated Press