Scottish and American prosecutors say they have no doubt about the guilt of the two Libyans charged in the bombing of Pan Am Flight 103.
Yet there are a swarm of alternate theories.
Some of the more provocative, published widely in the British and American press and on Internet sites, rival the conspiracy theories surrounding the John F. Kennedy assassination.
One such theory — vehemently denied by Washington — claims U.S. intelligence agents were smoothing the way for smugglers of Middle Eastern drugs to use Pan Am flights in exchange for information on American hostages in Lebanon. The terrorists allegedly slipped in the bomb with a covert drug shipment.
Vincent Cannistraro, who headed the early CIA inquest into the bombing, calls that theory "a tissue of fabrication," which he claims was planted by a private investigator to avoid insurance claims.
The theory with the strongest foundation is that Iran, rather than Libya, was behind the attack. That scenario says the bombing was retaliation for the 1988 downing of an Iranian airliner by a U.S. warship in the Persian Gulf, in which 290 people died.
Under that theory, Iran contracted out the Pan Am bombing to a financially strapped Syrian-based Palestinian group, the Popular Front for the Liberation of Palestine-General Command (PFLP-GC).
German police raids on PFLP-GC hide-outs in 1988 and 1989 yielded information the group was targeting a U.S.-bound flight from Frankfurt and turned up bombs that appeared to match the Lockerbie bomb.
"It's very difficult to believe there were two separate operations targeting the same flight," said Cannistraro, who believes the PFLP-GC handed off the bombing to Libya after the raids compromised its operation.
Another shadowy figure who has been linked to Lockerbie is Mohammed Abu Talb, who is serving a life sentence in Sweden for bombing American and Jewish targets in the Netherlands and Denmark. Abu Talb was sighted in Malta in the weeks before the bombing and Swedish police found a diary in his apartment with the date Dec. 21 circled.
Under Scottish law, defendants go free if their lawyers can raise the slightest doubt for the judges that someone else was responsible.