A THEORY GONE TO THE DOGS

By Kevin Fedarko

Ever since TWA Flight 800 exploded in midair on July 17, investigators from the National Transportation Safety Board and the FBI have been scrambling to determine what caused the Boeing 747 jumbo jet to plunge into the Atlantic, killing all 230 people on board. From the beginning, the theories--a missile, an exploding fuel tank, a mechanical failure or possibly even a meteorite strike--seemed almost as confusing as the jumbled pieces of the plane. And then a set of tantalizing clues emerged. Investigators discovered that several pieces of recovered wreckage bore minute traces of chemical explosives. That evidence provided the strongest support for a bomb theory. Last week, however, this line of inquiry was interrupted. On Thursday investigators learned that on June 10 St. Louis airport police had used the plane that would be TWA Flight 800 as a testing facility for a bomb-sniffing dog, and that the tiny amount of chemicals used to test the dog could be the source of the residue found on the plane parts.

With the bomb theory fading for lack of hard evidence, the focus of the NTSB investigation has returned to mechanical failure and the possibility that a spark, despite built-in safeguards, may have somehow caused the center fuel tank to explode. Again, the problem is evidence: there is none. Computer modeling--or the simulated explosion of a retired jumbo jet--may be required to buttress the theory.

How did the bomb theory go to the dogs? After finding traces of explosives on the wreckage, investigators ordered federal agencies to check into how the chemicals could have got on the plane other than via a bomb. One area examined was an faa program that requires the periodic recertification of bomb-sniffing dogs by placing real chemicals on empty planes. Finally, local airport records led to the St. Louis police canine unit.

The exercise involved placing "test packages of explosive chemicals" inside the plane so that the bomb dog could find them, according to the FBI. While the packages did not contain any substance that would actually blow up, they did contain chemicals such as PETN and RDX, both of which are building blocks for plastic explosives. The FBI says the packages were removed at the end of the tests, but the exercise could have left debris that may "possibly relate to the trace residues previously identified" on parts of the plane. Indeed, the PETN found on the floor of the passenger section and the RDX found at the back of the rear cargo compartment of the destroyed plane have been baffling investigators because neither deposit bears any evidence of "observable bomb damage" that might indicate the presence of an explosive device.

Given the way the investigation has gone, however, it could turn out the St. Louis bomb-dog test may have nothing to do with the mysterious residues in the plane. Nevertheless, this latest wrinkle makes any bomb investigation even harder. "This discovery would definitely hurt any attempt to build a circumstantial case of a bomb," a source close to the investigation told the Associated Press. "Now we would definitely need pitting or blast damage to prove there was a bomb placed on board."

--By Kevin Fedarko. Reported by Elaine Rivera/New York and Elaine Shannon/Washington

With reporting by ELAINE RIVERA/NEW YORK AND ELAINE SHANNON/WASHINGTON