WASHINGTON -- The clue that turned the case was a microchip, a tiny piece of a triggering device used to detonate a bomb.
From it, U.S. and Scottish investigators discovered a new trail that refuted the conclusions of almost two years of arduous legwork by thousands of agents worldwide -- and altered major assumptions about the downing of Pan Am Flight 103 over a small Scottish village four days before Christmas 1988.
The key breakthrough was almost a fluke. A "brilliant young CIA analyst," as one insider described him, decided to test a new hypothesis: Could someone besides the widely suspected culprits -- Palestinian radicals, their Syrian patrons or Iranian militants -- have been involved?
The analyst started with a hunch.
He searched for a "signature" that would match the Pan Am bombing with earlier incidents. Culling through CIA files, he came up with the 1984 bombing of a French UTA airliner in Chad. A premature explosion blew up the baggage compartment while the plane was still on the ground, wounding 27 people.
He also found a link with the 1986 attempt to blow up the U.S. Embassy in Togo. Officials in Lome, the Togolese capital, had arrested nine people with two suitcases full of plastic explosives.
But the biggest find was an obscure case in Senegal involving the arrest of two men at an airport in Dakar in February 1988. In their possession were 20 pounds of sophisticated Semtex plastic and TNT explosives, weapons and several triggering devices. The suspects later were released.
In all three cases, the signature was distinctly Libyan.
The two men who were arrested in Senegal -- Mohammed Marzouk, alias Mohammed Naydi, and Mansour Omran Saber -- were both Libyan intelligence agents. The triggering devices in their possession matched the microchip fragment from the Pan Am bomb.
Based on the forensic evidence and the links with earlier cases, investigators now believe:
* The regime of Col. Muammar el Kadafi carried out the bombing. Libyan intelligence, headed by Abdullah Sanussi, orchestrated the plot.
* The primary motive was revenge for the 1986 U.S. bombing of Tripoli, in which about 40 people, including Colonel Kadafi's infant daughter, were killed. "The notion that the 1986 bombing of Tripoli deterred Libyan terrorism is greatly flawed," concludes a leading counterterrorism expert.
* The mysterious bag carrying the bomb-laden Toshiba cassette player on Pan Am 103 came from Malta. Investigators believe the bomb was probably flown on an Air Malta flight to Frankfurt, Germany -- although the passenger and cargo log has disappeared. In Germany, the cassette player was loaded onto Pan Am 103 as an interline bag, unattached to any passenger.
The new evidence means that, unlike the unsolved cases of a half-dozen terrorist attacks against U.S. targets in the 1980s, the bombing of Pan Am Flight 103 may go to court.
Assistant Attorney Gen. Robert S. Mueller III, who heads the Department of Justice's Criminal Division and has been meeting frequently with the FBI on the investigation, appears ready to take the case to a grand jury, according to U.S. officials.
Should the grand jury return sealed indictments, the biggest obstacle may not be arresting those involved. U.S. authorities already are working with French police seeking to apprehend one of the Libyan suspects somewhere in North Africa, officials say.
The biggest problem may be competition over which country will get them for trial. French intelligence now believes another terrorist attack -- the 1989 bombing of UTA flight 772 over Niger -- was also directed by Libyan intelligence.
The new evidence, which began to emerge last summer, contradicted the long-standing belief that the Lockerbie bombing was linked to the Popular Front for the Liberation of Palestine-General Command headed by Ahmed Jibril. The PFLP-GC, a radical group based in Syria, is outside the PLO umbrella.
The original case was based on the arrest of a cell of 16 operatives in Germany two months before the 1988 bombing. The group was found to have five bombs, specially designed to blow up aircraft, hidden in electronic equipment.
From his base in Damascus, Mr. Jibril was also known to have worked closely with Iran, where he frequently traveled. Investigators believed Tehran commissioned the PFLP-GC to target a U.S. plane in retaliation for the accidental 1988 U.S. downing of Iran Air Flight 655 over the Persian Gulf, in which 290 persons died.
The crucial clues that changed the direction of the probe were the detonators. The PFLP-GC's detonators were all Czech-made. The detonator fragment that was culled from the wreckage of Flight 103, which had been scattered over 845 square miles of Scottish countryside, was of Swiss manufacture -- from the same company that had made the triggering devices that were found on the Libyan agents in Senegal.