Presidents, princes, even the pope got into the act of bringing the two Libyans accused of blowing up Pan Am Flight 103 to justice.

After interviewing 15,000 witnesses and collecting 180,000 pieces of evidence in the biggest murder investigation in history, prosecutors are convinced they have the culprits. 

But the prosecution's case has suffered from the many compromises struck to set up the Lockerbie trial in the Netherlands. Also, according to Scottish legal experts, the prosecution has no eyewitness who can establish incontrovertibly that the defendants planted a suitcase bomb aboard the doomed airliner. The rigorous standards of proof in Scottish law require that every incriminating fact be backed up by more than one source. 

A host of questions remain unanswered: 

— How did the suitcase bomb pass unaccompanied through two aircraft changes — in Frankfurt, Germany, and London — without raising suspicions? 

— Why did warnings received by U.S. embassies, including one specifically mentioning a U.S.-bound flight from Frankfurt, fail to prevent the tragedy? 

— Did the Libyans act alone? 

Such riddles have spawned a plethora of conspiracy theories in press reports and Internet sites and dampened the hopes of victims' relatives that justice will be served. 

"We may be farther from justice after this trial is over than beforehand," said Dan Cohen, whose daughter Theo was also returning from a European study program on Flight 103. 

The Explosion 

At 7:02 p.m., as the airliner was about to turn out over the Irish Sea, a bomb inside a brown Samsonite suitcase blew a hole in the fuselage. 

Within seconds, the Boeing 747 ripped apart and a downpour of carnage — bodies, luggage, mangled metal — blanketed lawns and streets of the town of Lockerbie, Scotland. 

The evidence was scattered over 845 square miles of southern Scotland, in the world's worst act of air terrorism. 

Investigators have relentlessly pursued a trail of clues from the verdant Scottish hills to the sunny Mediterranean island of Malta, where the bomb purportedly began its fateful journey. 

It was a fingernail-sized fragment of a circuit board from a timing detonator found in a Scottish forest that led investigators to the Libyan connection. 

The fragment was cited as the key piece of evidence when U.S. and British authorities announced the indictment of Abdel Basset Ali al-Megrahi and Lamen Khalifa Fhimah on Nov. 14, 1991. 

Suspecting Libya 

From that moment, the bombing was portrayed as a Libyan operation. Washington and London demanded that the country's leader, Moammar Qaddafi, hand over the men for trial. 

Suspicions previously had focused on Iran and a terrorist group sponsored by Syria, the Popular Front for the Liberation of Palestine General Command (PFLP-GC). 

"A lot of the indications pointed toward the PFLP-GC carrying it out as a commission for the Iranians," said Vincent Cannistraro, who headed the CIA's Lockerbie probe in the early years. 

Around six months before the bombing, a U.S. warship had shot down an Iranian Airbus over the Persian Gulf, killing all 290 occupants. Tehran's leaders swore revenge. 

One of the strongest indications of possible PFLP-GC involvement was a bomb discovered by West German police in an October 1988 raid in Neuss. 

Like the Lockerbie bomb, it was built into a Toshiba tape recorder. The man who allegedly built it had been seen carrying a brown Samsonite suitcase. He confessed to having built four more bombs, only three of which were recovered. 

But investigators say the PFLP-GC bombs were triggered by an altitude detonator — not a timer, like the Lockerbie bomb. 

Investigators traced the timer fragment to a Swiss firm whose owner admitted he sold a batch to Libya in the mid-1980s. 

The Standoff 

In 1992, the U.N. Security Council imposed sanctions on Libya trying to force the surrender of the defendants. Qaddafi refused, contending the men would not get a fair trial in Britain or the United States. 

Toward the end of the decade, pressure for a compromise increased as cracks appeared in observance of the sanctions. 

South African President Nelson Mandela, Pope John Paul II and Arab leaders all called for an end to sanctions. U.N. Secretary-General Kofi Annan asked Mandela and Saudi Crown Prince Abdullah to help mediate a settlement. 

In 1998, the United States and Britain reached an agreement with Qaddafi for the two suspects to stand trial in an extraterritorial Scottish court proceeding that was unprecedented in international law. 

On April 5, 1999, al-Megrahi and Fhimah were extradited to the Netherlands to stand trial before a court led by Lord Sutherland, a Scottish High Court judge. 

Case Not Ironclad 

Prosecutors say the defendants are Libyan intelligence agents who worked for Libyan Arab Airlines on Malta and planted the suitcase bomb on a flight originating on that island nation off Libya's Mediterranean coast. 

The unaccompanied suitcase allegedly was transferred at Frankfurt to Pan Am Flight 103-A to London, where it entered the cargo hold of the doomed jetliner. 

Questions have been raised about that version. 

The owner of the Swiss electronics firm later recalled that he also sold the same type of timer to the former East German security service, the Stasi, which used to supply the PFLP-GC and other terrorist groups. 

A Libyan defector who claimed to have seen one of the defendants put the suitcase on the luggage carousel at Malta's Luqa Airport retracted the story under questioning from defense lawyers, Scottish legal sources say. 

And the proprietor of a Maltese boutique who identified Al-Megrahi as the buyer of clothing that was in the bomb suitcase, later confused him with Mohammed Abu Talb, a PFLP-GC suspect now serving a life sentence in Sweden for terrorist bombings. 

Sources say defense lawyers plan to raise Abu Talb's possible role and have evidence to point to other potential conspiracies. 

One such theory claims U.S. authorities were allowing Middle Eastern drugs to be smuggled onto Pan Am flights in exchange for information on American hostages in Lebanon. Terrorists allegedly took advantage of the drug shipments to slip a bomb onto the flight. 

U.S. officials deny any such shipments were occurring. 

Despite all the effort put into the investigation, prosecutors have appeared to be stumbling. 

Scottish Chief Prosecutor Lord Hardie unexpectedly resigned in February. And this past week prosecutors asked for a two-month postponement, blaming the defense for submitting witness lists and evidentiary exhibits at the last minute. 

Sutherland rejected the request Thursday and ruled the trial would begin this Wednesday as scheduled. It is expected to last a year. 

Robert Black, an Edinburgh University law professor and Lockerbie native who first proposed a trial in the Netherlands, is one who has doubts about prosecution's case. 

"The closer one gets to the actual start, the less likely there appears to be the chance of a conviction," he said. 

—The AP contributed to this report