IF IT WAS A BOMB, THEN WHODUNIT?

By CHRISTOPHER JOHN FARLEY

The ocean stubbornly held on to its mysteries for most of last week. It was especially uncooperative on Thursday for the men and women investigating the crash of TWA Flight 800. Rain and strong winds disrupted and eventually interrupted search operations. But on Friday there was a breakthrough. Deep Drone 7200, a remotely operated robot outfitted with cameras that can explore ocean depths without divers, located part of the cockpit, "the nerve center of the aircraft," as Robert Francis, vice chairman of the National Transportation Safety Board (NTSB), described it. Said James Kallstrom, the FBI's lead investigator: "I just think that somewhere in the front of the plane is a clue." Investigators generally believe that if a bomb destroyed Flight 800, it exploded in the front of the aircraft. Examining the cockpit could help prove that theory.

As the Navy continued to search for and recover bodies, investigators have begun to speculate not only about the cause of the disaster but also about the originators of what they are almost certain is a monstrous crime. A well-placed U.S. intelligence source has told TIME that calls and transmissions tracked by the CIA out of Tehran "have raised suspicions" that there is an Iranian connection to the crash. The CIA is also looking at intelligence on a meeting of terrorist leaders in Iran the month before the crash to see if any green light was given for the attack. "There's a hard look being taken at the Iran possibility," says a senior U.S. intelligence official. However, he adds, the intelligence gathered so far is "vague, nothing solid." Even so, he says, it is "tantalizing."

FBI agents, working closely with intelligence agencies here and abroad, are scouring all incoming reports about a possible Iran connection. Of interest are the recent movements by an alleged Hizballah terrorist named Hussein Mikdad, who is purportedly backed by Iran. On April 4, Mikdad took a Swissair flight from Zurich to Tel Aviv with bomb parts hidden in a carry-on bag. Eight days later, Mikdad blew off both his legs and one arm when a bomb he was assembling in an east Jerusalem hotel room accidentally detonated. Israeli security officials believe Mikdad was building the bomb using a powerful plastic explosive called RDX and planned to place it aboard a flight leaving Ben Gurion Airport. This was the first known time that Hizballah slipped an operative into Israel by way of an international airline. FBI agents are planning to travel to Israel to study Mikdad's methods for any telltale bits of bombcraft that may be traced to TWA Flight 800.

The Iranian links to terrorism were further highlighted last week when Defense Secretary William Perry, in a National Public Radio interview, hinted that an ongoing Saudi investigation of the June 25 bombing of a U.S. military complex in Dhahran may "possibly" point to Iran's involvement. He suggested that the U.S. might have to consider "strong action."

In the TWA Flight 800 case, federal points of inquiry already assume the existence of a crime. If there was a bomb, did it contain Semtex, a powerful Czech explosive used by Libyan agents in the Pan Am 103 bombing? Or a lower-grade nitroglycerine-nitrocellulose mix? And what evidence can be extrapolated from the existing clues to help answer these questions?

Those clues are being arduously analyzed by experts at the NTSB and the FBI. One cluster of NTSB engineers is tracking the trajectories of pieces of wreckage from where they landed to where they began to fall. This is done with computers that factor in radar records, wind direction and speed, and other data. The studies will help experts determine the sequence of catastrophic events that led to the plane's destruction. Also, the sharp sound at the end of the cockpit voice recording is being analyzed in minuscule detail, with attention to the different speeds at which the vibrations travel through air and metal. If the gap between the sound's arriving by air and by metal is small, that means the event was close to the cockpit because the vibrations did not have time to gain a large lead.

One conjecture being explored by some government counterterrorism specialists is that the blast was caused by a "fuel-air explosion," probably indicating that a low-grade explosive device was involved. This theory, so far a minority view, holds that an explosion would cause fuel to leak into the air and then be ignited by the slow-burning detonating material, creating what amounts to a giant gas bomb; a higher-velocity explosive like Semtex would cause severe structural damage to the plane, but the intense blast might be too short to ignite fuel vapors

FBI explosives-unit chief Tom Thurman and his associates will be looking for what is called "observable bomb damage" in the wreckage. Says Chris Ronay, a retired FBI agent who preceded Thurman as head of the unit: "They call us the blacksmiths of the laboratory. We don't use too much sophisticated instrumentation. We use hammers and trowels and microscopes." Members of the unit will be studying the plane's metal surfaces for tiny pitting that could have been caused by an explosion that would have melted tiny pieces of metal and sent them, and other tiny bits of debris, shooting around like buckshot. Says a bomb specialist: "It's a high-speed phenomenon, faster than the speed of a plane crashing."

Much of what the investigators seek remains beneath the ocean, in waterlogged seats, shreds of clothing, mangled bags and carpet scraps. Every bomb scatters tiny bits of undetonated explosive, and some of those particles penetrate porous material and lodge there, protected from the elements. Says a bomb specialist: "Nylon, porous materials, seat cushions--a lot of stuff can get in there and stick. You wash down the debris with a solvent and run it through the machines." Crash clues, in the end, could come in very small packages. Says an aviation expert: "All the significant evidence could fit on top of a desk." But finding that evidence could require raising most of the plane. Through Friday, less than 10% of the aircraft had been recovered.

--Reported by Elaine Rivera/Long Island and Elaine Shannon and Douglas Waller/Washington

TIME's TWA crash site at time.com/twa offers a special report on the disaster and daily news updates

With reporting by ELAINE RIVERA/LONG ISLAND AND ELAINE SHANNON AND DOUGLAS WALLER/WASHINGTON