The theories

If Megrahi's appeal against his conviction is upheld, one glaring question will remain: who is really responsible for the deaths of the 270 Lockerbie victims?

Many theories - some plausible, some outright ludicrous - have been proposed over the past 19 years.

They range from the late Ayatollah Khomeini of Iran ordering the bombing, to CIA involvement. The SCCRC dispelled some of the more enduring suggestions yesterday, including the allegation that Scottish police officers were involved in tampering with evidence.

One of the conspiracy theories came from a former detective sergeant involved in the police investigation. His identity has never been revealed, and he is only ever referred to by the codename "Golfer".

He alleged that evidence was fabricated and planted to create the Maltese chain of evidence. In particular, he claimed fragments of bomb-damaged clothing, an instruction manual for a Toshiba recorder, and parts of a timer circuit board were all planted to implicate Megrahi.

SCCRC investigators interviewed him three times, and concluded his claims were without substance.

It said: "There was a vast array of inconsistencies and contradictions between, and sometimes within, his statements in light of this, the commission has serious misgivings as to the credibility and reliability of this witness."

Further allegations that statements, evidence, and other records were manipulated, altered, and fabricated were dismissed by the SCCRC, which said there was "no proper basis" for any such claims.

Perhaps the most controversial conspiracy theory is that the CIA, US Military intelligence, or the Defence Intelligence Agency (DIA), and the Israeli secret service were behind the bombing.

The CIA is alleged to have been selling drugs to raise funds to buy the freedom of six US hostages held by Hizbollah in Lebanon at the time. The drugs-ring and the connection to Hizbollah is said to have been set up by Israeli Mossad agents.

It is alleged that PanAm flights were used to courier the drugs, but on one of the runs the drugs were supplanted by a bomb.

The theory is that the bomb's target was one of the passengers, Major Charles McKee, a US intelligence expert who had discovered the drug-trafficking ring.

The SCCRC investigated a claim made by a former police officer involved in searching for debris after the crash that he found a CIA badge, only to be told by colleagues not to record it as evidence.

This was discounted after the investigators interviewed another officer allegedly present at the time. They also rejected the allegation that evidence was "spirited away" by the CIA, including Major McKee's suitcase.

The commission said it found "no evidence to suggest that anyone other than Scottish police officers came into contact with Major McKee's suitcase at the scene of the crash".

Numerous allegations have been made that one of the Lockerbie victims, Khaled Jaafar, an American national of Lebanese extraction, had been used to take the bomb on board the flight at Frankfurt.

Jaafar has been accused of being involved in the alleged CIA-approved heroin-smuggling operation. The SCCRC said it found "no support for the claim that Mr Jaafar was involved, wittingly or unwittingly, in the bombing".

Iran's alleged motive for carrying out the attack was assumed to be a desire for revenge for the shooting down of an Iranian civilian flight by the US warship, the Vincennes, killing all 290 passengers and crew aboard, in July 1988. Ayatollah Khomeini, then supreme leader of Iran, vowed at the time that the skies would "rain blood" in revenge.

Those who maintain that Iran was behind the attack say Tehran sponsored one of a number of radical Palestinian groups to carry out the attack. The group alleged to have executed the plot was the Syrian-backed Popular Front for the Liberation of Palestine-General Command (PFLP-GC), headed by Ahmed Jibril.

During a raid by German police on one of Jibril's PFLP-GC cells in Dusseldorf in 1988, during which 14 people were arrested, four Toshiba cassette recorder bombs were discovered. At Megrahi's trial, the Crown argued the type of bomb used to blow up Flight 103, while encased in a similar Toshiba recorder, used a different triggering mechanism. Jibril was assassinated in Beirut in 2002.

Egyptian-born Abu Talb, who was allegedly working for Jibril, emerged as a key suspect in the aftermath of the bombing. However, when Megrahi went on trial in 2000, Talb was called as a witness for the prosecution.

New information regarding Talb and his activities in Europe was sent to the SCCRC during its investigation. It is alleged he was paid millions of dollars by the Iranian government following the bombing. Talb is accused of getting the bomb, made by Marwen Khreesat, a master bomb-maker for the PFLP-GC, on to the flight. He is currently serving a life sentence in Sweden, having been convicted in 1989 of bombing a synagogue in Denmark in 1985.

The SCCRC did not make any comment on the allegations concerning Talb yesterday.