SECRET DRUGS, THE SUITCASE AND THE CIA
THE CIA is alleged to have set up a protected drug route from Europe to the United States - called Operation Corea - which allowed Syrian drug dealers to ship heroin to the US using Pan Am flights, in exchange for intelligence on Palestinian groups based in Syria.
The CIA allegedly protected the suitcases containing the drugs to guard them from searches.
On the day of the bombing, as the theory goes, terrorists exchanged suitcases: one with drugs for one with a bomb.
A more extreme version of this theory is that the CIA knew in advance this exchange would take place but let it happen anyway, because the protected drugs route was a rogue operation, and the American intelligence officers on Pan Am 103 - Matthew Gannon and Major Charles McKee - had found out about it, and were on their way to Washington to tell their superiors.
The former version of the protected-suitcase theory was suggested in October 1989 by Juval Aviv, the owner of Interfor Inc, a private investigation company based in New York. Aviv was an ex-Mossad officer and was employed by Pan Am as their lead investigator for the bombing.
His claims provided Pan Am with a credible defence against claims for compensation by relatives of victims, since, if the US government had helped the bomb bypass Pan Am's security, the airline could hardly have been held liable.
Aviv was never interviewed by either the Scottish police or the FBI.
THE IRANIAN THEORY
THE most enduring alternative theory behind the Lockerbie bombing involves Iran, a Palestinian terrorist group and a convicted bomber called Muhammad Abu Talb.
Its roots lie in the accidental downing by a United States warship of an Iranian Airbus in the Persian Gulf on 3 July, 1988, which killed 300. Days later, an Iranian intelligence officer is said to have flown to Lebanon to meet two officials from the Popular Front for the Liberation of Palestine - General Command (PFLP-GC), Muhammad Hafiz Dalkamoni and a man known to the CIA as "Nabil".
The theory has it that the Iranian told Dalkamoni and Nabil to blow up a US plane, in the air, in order to kill as many people as possible.
Iranian hardliners never accepted that the Airbus was shot down accidentally by the USS Vincennes. Iran had also developed a taste for letting surrogates do its dirty work, and the PFLP-GC was one of the best in the world when it came to blowing things up.
Dalkamoni appeared to have a watertight alibi for Lockerbie. He had been arrested with most of his German cell on 26 October, 1988, and was still in custody when the plane exploded two months later.
But former CIA officer Robert Baer claims there was an "avalanche of information" pointing to the possibility that the operation had been handed over to one of the cell members who got away.
Two days after the bombing, an $11 million (5.5 million) transfer showed up in a PFLP-GC bank account in Lausanne, Switzerland. It moved from there to another GC account and the Banque Nationale de Paris. The Paris account number was found in Dalkamoni's possession upon arrest.
Also, it is understood that Muhammad Abu Talb, one of Dalkamoni's associates suspected of having a role in the bombing, received a payment of $500,000 (250,000) on 25 April, 1989. That raised the question: was the money a reward for Pan Am 103?
Mr Baer provided evidence to the Scottish Criminal Cases Review Commission.
LIBYA AND ABU NIDAL
IN THE early 1980s, Abu Nidal was widely regarded as the most ruthless international terrorist - until that mantle was assumed by Osama bin Laden.
Nidal was reported to have died in a shoot-out in Baghdad on 16 August, 2002. A former senior member of his group, Atef Abu Bakr, told journalists that shortly before his death, Nidal had confided to Bakr that he had orchestrated the Pan Am Flight 103 bombing on behalf of Libya's Colonel Gaddafi.
After settling in Tripoli in 1985, Nidal and Gaddafi allegedly became close, Gaddafi sharing what the Sunday Times called "Abu Nidal's dangerous combination of an inferiority complex mixed with the belief that he was a man of destiny".
According to Atef Abu Bakr, Col Gaddafi asked Nidal to co-ordinate, with the head of Libyan intelligence Abdullah al-Senussi, an attack on the US in retaliation for the 1986 bombing of Benghazi and Tripoli.
Nidal then organised the hijacking of Pan Am Flight 73 in Karachi on 5 September, 1986, killing 22 passengers and wounding dozens of others.
For Pan Am 103, Senussi allegedly told Nidal to supply the bomb, and Libyan intelligence would arrange for it to be put on a flight. No evidence has been produced in support of this theory.
SOUTH AFRICA LUGGAGE SWAP
ACCORDING to this theory, apartheid South Africa was responsible for the sabotage of Flight 103.
On 22 December, 1988 - the day after the Lockerbie bombing - the Namibia independence agreement was signed at UN headquarters in New York.
A 23-strong South African delegation, headed by foreign minister, Pik Botha, cancelled a booking on the flight at short notice.
There was also a last-minute change of travel plan by the UN Commissioner for Namibia, Bernt Carlsson.
Carlsson is alleged by some to have been the target of South African military intelligence operatives, having been the architect of Namibian independence.
Instead of flying direct from Brussels to New York, Carlsson was persuaded to stopover in London and join the Pan Am 103 transatlantic flight.
On the day of the bombing, he arrived at Heathrow from Brussels at 11:06 with a booking to travel onward to New York by flight PA 103 at 18:00.
Carlsson was met at the airport by Bankole Timothy of De Beers and taken by car to central London.
After the meeting with De Beers, Carlsson was brought back to Heathrow Airport, arriving at about 17:30.
His already checked-in suitcase would have remained at Heathrow airport for about seven hours, thus providing South African airside-authorised personnel with ample opportunity to substitute it for the bomb suitcase.
That South Africa Airlines were involved in unlawfully switching baggage that day was confirmed by a Pan Am security officer, Michael Jones, at the Lockerbie fatal accident inquiry in October 1990.
Within a week of the death of Bernt Carlsson on flight PA 103, his office safe at the United Nations had allegedly been broken into. And his apartment, which had been sealed by UN security staff, had also apparently been burgled.
Neither his girlfriend nor his sister could identify a single shred of anything in his luggage at the property store in Lockerbie.