NEW YORK — Casting doubts on the theory that a bomb exploded aboard TWA Flight 800, killing 230 people, the FBI announced Friday that the plane had carried test packages of explosives during exercises to train bomb-detection dogs the month before the crash.
The FBI said the training was part of an overall Federal Aviation Administration security exercise conducted along with airport police in St. Louis in June. That, the FBI said, could explain why traces of explosives were found in the aircraft's wreckage. Investigators so far have been unable to back up these laboratory findings with any of the classic signs of bomb damage, which include metal that is pitted or bent by high energy shock waves.
With 80% of the plane now recovered and being reassembled in a hangar at Calverton, N.Y., attention is increasingly being focused on the Boeing 747 jumbo jet's center fuel tank as the site of a possible fatal malfunction.
Experts are trying to determine whether an accidental spark may have ignited a highly volatile mixture of air and jet fuel in the tank.
"The Federal Bureau of Investigation has learned that within the last year, the airplane used in TWA Flight 800 had previously been used in a law enforcement training exercise for bomb-detection dogs," the agency said in a statement.
"As part of a routine recertification test for a dog, test packages containing explosive chemicals were placed aboard the aircraft, which could possibly relate to the trace residues previously identified. The test packages were removed from the plane at the conclusion of the exercise."
A federal investigator said that such packages commonly contain PETN, an explosive component found in many bombs and surface-to-air missiles, because trainers want dogs to learn to find a "smorgasbord" of explosive ingredients.
Chemists at the FBI's crime laboratory in Washington, as well as agents in the Calverton hangar confirmed the presence of microscopic specks of PETN in widely separate pieces of the plane recovered by divers off the coast of Long Island, N.Y., where the jet crashed July 17.
Chemists at the FBI's crime But the dog-training exercises could provide "another explanation for those positive hits," the federal crash investigator said. "It suggests an alternative."
TWA said that it could not comment on the training exercises and referred all calls to the FBI.
However, a source at the airline said that the dog training was conducted under the auspices of the FAA and that TWA often cooperates with law enforcement on training to detect explosives.
The FBI and the National Transportation Safety Board were notified Friday by the FAA about the training exercises, sources close to the investigation said. When asked why the information had not been passed along to other agencies sooner, a federal source said: "The guys in [FAA] security don't tell anybody about anything."
An FAA official said the agency's investigators found records late Thursday showing that the plane had been used for testing dogs on their bomb-detection ability. Officials began looking for the match about a month ago.
"It was a long tedious process of matching FAA and TWA records," the official said, speaking on the condition of anonymity.
Before learning that the plane was used to train bomb-sniffing dogs, investigators had been looking into the possibility that the minute specks of PETN could have been brought on board by U.S. troops who used the aircraft for transportation to the Persian Gulf.
Because the plane plunged into the ocean soon after takeoff, investigators have concentrated on three theories--that either a bomb, a missile or mechanical failure caused the crash.
These investigators stressed that while the dog-training exercises could diminish the bomb theory, it by no means ruled it out. The remaining 20% of the plane still under water could contain definitive evidence of bomb damage, and plans are being considered to recheck the pieces of the aircraft that have already been recovered for signs of explosives.
A series of setbacks, including storms that have periodically forced divers from the water, has slowed recovery efforts, and investigators admit that the delay has in all likelihood degraded or even destroyed explosive residue possibly coating plane parts still beneath the Atlantic Ocean's surface.
Regardless of the cause, both FBI agents and National Transportation Safety Board investigators are concentrating on the center section of the aircraft. Only about one-fifth of that portion of the plane has been found so far, and investigators theorize it was the site of the initial explosion.
Experts are eager to examine the plane's third fuel pump, if it can be found. Divers recovered from the suspect center fuel tank two pumps, both of which were analyzed for mechanical malfunction. No abnormalities were discovered.
The FAA, however, is considering requiring "repetitive visual inspections" of fuel pumps aboard Boeing 747 and series 757 airplanes after receiving reports of leaking fuel pumps as well as moisture in the pumps that leads to corrosion of wires.
The FAA noted that such corrosion can cause electrical fires.
Times staff writer Eric Malnic in Los Angeles contributed to this story.