Traces of nitrate near the “blast seat,” or epicenter of the explosion, helped convince investigators that a bomb–not a ruptured gas line or other accident–caused the damage. They will continue to search for other corroborating evidence: chemical residue, shards of plastique, slivers of wire, timer parts and electrical material. This case differs from the investigation into the bombing of Pan Am Flight 103, which spread debris across 845 square miles of countryside. The evidence here is confined to the World Trade Center-but some inevitably was destroyed in the evacuation effort.
Like a silversmith’s hallmark the bomb’s “signature,” or unique style, helps identify its creator. FBI counterterrorism experts will look for such tags in the debris: the presence of a particular device could lead investigators to the terrorist group associated with it in past explosions. Once officials identify the component parts and trace them to their manufacturers, the FBI will work with local or international law-enforcement agencies to determine who purchased the materials. In the Pan Am case, a shard of circuit board the size of a fingernail and a fragment of a timer helped investigators finger two Libyan agents.
The circumstances of the bombing provide clues to the sophistication of the operatives. In this case, the bomber obviously had detailed intelligence on the World Trade Center and knew where to place the explosives for maximum effect. By choosing a subterranean garage, the attacker magnified the effect of the blast and directed the explosive force up into the center of the structure. “You would need a bomb 10 times as big to do the same damage outside the building,” says one State Department counterterrorism official. “It looks like the people who did this had training somewhere.” At this early stage of the probe, experts disagree about just how sophisticated the bomber or bombers actually were. One government bomb expert estimates that the device consisted of 400 pounds of plastique. In a trade whose pros use just the right amount to do the job, he says, “this was overkill.”
The FBI had no intelligence indicating preoperational planning by any terror organization. Now the bureau is searching computer databanks for information that seemed insignificant before the attack but now appears relevant. Computer analysis will play a key role in solving the mystery.
