By Paul Lashmar 

THERE IS only one indisputable truth about PanAm 103, which is that at 19.02 on 21 December 1988, a Boeing 747 exploded at 31,000ft and crashed on Lockerbie, killing 270 people. Since then, myriad allegations have reached critical mass, allowing a variety of conspiracy theories to be constructed. 

And they all sound plausible: the Libyans did it to avenge Ronald Reagan's attack on Tripoli in 1986; the Iranians to avenge the accidental shooting down of one of their airliners by the USS Vincennes in July 1988; Palestinian terrorists did it to eliminate a CIA team preparing to rescue Western hostages in Beirut. 

The official finger points at the Libyan leader, Muammar Gaddafi, but the international spotlight focuses on two small-time Libyans, Abdel Basset al-Megrahi and al-Amin Khalifa Fhimah, accused of being agents who planted the bombs for Libyan intelligence. Charges were laid in 1991 after an investigation led by Chief Inspector Watson McAteer of Dumfries and Galloway Police, assisted by the FBI. Investigators believed the bomb was 10-14oz of Semtex in a Toshiba cassette recorder, itself in a brown Samsonite suitcase. 

At the time, both Libyans were working for Libyan Arab Airlines in Malta. The prosecution will claim they smuggled a bomb in an unaccompanied case into the airport's luggage transfer system and the case was transferred to PanAm 103 at Heathrow. 

The key evidence hinges on a fragment of circuit board found in baggage of the plane. It is believed to be part of a timing device sold by a Swiss company to Libya. Tags on clothes in the case proved they were Maltese, sold in only one shop there. The owner was shown photos of Arabs and is said to have picked out Mr Megrahi. In statements he said the man was older and taller than Mr Megrahi. More recent evidence confirming Colonel Gaddafi's hand in the affair comes from a mysterious Libyan defector. 

But, seven years on, the central planks of the prosecution case look shaky. The Maltese authorities and airline do not accept that unidentified luggage left Malta. The circuit boards were also sold elsewhere. Such are the uncertainties that those with a close interest in the case, including Tam Dalyell MP and Jim Swire, have doubts about the case against the Libyans. 

It is now suspected that Libya was scapegoated. Mr Dalyell suspects Iran, aided by Syria, might be the guilty party. The official accusation against Libya came shortly after the Gulf war. Here, says Mr Dalyell, is the clue: "The West wanted Syria and Iran to be benevolent towards military action against Saddam Hussein." 

The US itself muddied the waters of the inquiry within hours of the bombing: mysterious officials were reported on the crash site, tampering with bodies and evidence. 

The Iranians had a specific motive and long-standing animosity towards the US. According to one theory, after the Vincennes shot down the Iranian Airbus, the interior minister, Ali Akbar Mohtashemi, and hard-liners commissioned the Syrian-based Popular Front for the Liberation of Palestine, General Command, led by Ahmed Jabril, to carry out the revenge attack. 

His bomb-maker, Marwan Khreesat, was sent to Germany and assembled five bombs. But in October he and 14 other suspects were arrested and a bomb was seized. 

Members of the unit, including Khreesat, were later freed by the Germans, which raised the question - was he a double agent? 

Five weeks later, US and British authorities were warned that a bomb was to be placed on a US transatlantic flight. It is said VIPs and US diplomats and CIA staff cancelled bookings on PanAm 103, enabling late bookers such as Flora Swire to get a seat. 

The next step of the theory was most clearly postulated by the late US documentary- maker Allan Francovich in his Maltese Double Cross. The argument centred on a Lebanese-born American, Khalid Jafaar, who was on PanAm 103. Part of a drug-producing family from the Bekaa valley, he was ordered by Hizbollah to take heroin to the US. But Jabril's men switched his heroin for the bomb when he stopped in Germany. 

This theory says Jabril was taking advantage of a deal struck by the CIA and the US Drug Enforcement Administration with the Syrian bosses of Lebanese drug-trafficking. In return for helping the Syrians use their influence to free the remaining American hostages, the CIA helped them transport heroin to the US. Jabril used this arrangement to get the bomb on to the aircraft. 

Adding fuel to this theory was the presence on PanAm 103 of Major Charles McKee, who, unannounced, was returning with his CIA team to the US from Beirut. He was part of an operation to free the Lebanon hostages. The sub-plot is that his team were killed to prevent key information getting back to the US. 

If you think this is complicated, remember it is the simplified version. For once the cliche "in a plot worthy of John Le Carre" does not do the reality justice. Even if the two Libyans are tried, the real truth may never be revealed. 

Ten years of investigating Lockerbie 

21 Dec 1988: Bomb explodes on PanAm 103 flying at 31,000ft, killing all 259 on board and 11 more as part of jet crashes on Lockerbie. 

31 Jan 1989: US government reveals more than 100 flaws in airport security. 

1 April: Lockerbie victims' committee called UK Families Flight 103 set up. 

10 May 1990: PanAm reaches out-of-court settlement with Scottish families of dead. 

16 Oct 1991: US Supreme Court rejects appeal for punitive damages against PanAm by relatives of victims. 

14 Nov: Charges against Libyans Ali Basset al-Magrahi and al-Amin Khalifa Fhimah announced in Washington and Edinburgh. 

15 Nov: Libya calls for international inquiry, offers to send dispute to neutral bodies. 

22 Jan 1992: UN resolution urges Libya to comply with extradition or face sanctions. 

31 March: UN tells Libyan leader Colonel Muammar Gaddafi to surrender suspects by 15 April or face worldwide ban on air travel and arms sales. 

15 April: UN air and arms embargo takes effect. 

8 April 1993: UN retains sanctions against Libya. 

13 August: US, France and Britain threaten Libya with tougher sanctions by 1 October. 

1 Oct: UN deadline for Libya to surrender the two suspects for trial expires. 

2 Oct: Britain, France and the United States stiffen sanctions. 

17 Feb 1994: Britain rejects offer by Libya to allow suspects to stand trial before Muslim court anywhere in the world. 

23 March 1995: US offers $4m reward for information leading to arrest. 

Nov: Families secure out-of-court settlements from PanAm's insurers. 

April 1996: Libyan suspects agree to trial under Scottish law in neutral location. 

10 July: UN sanctions begin to weaken. 

Oct: Victim's father Jim Swire meets South African President, Nelson Mandela. 

25 Oct: At Commonwealth conference in Edinburgh, Mr Mandela urges Britain and US to accept a trial in neutral state. 

28 Oct: In Commons, Tony Blair insists Libyans should stand trial in Scotland. 

20 April 1998: Mr Swire travels to Libya with Scottish law expert Professor Robert Black to broker terms of trial. 

18 Aug: BAe reported to be negotiating aircraft deal with Libya. 

Al-Amin Khalifa Fhimah 

Mr Fhimah was listed as the airline station manager at Luka international airport in Malta for the Libyan Arab Airlines. 

But Western intelligence says that is a cover for his real occupation, as a Libyan intelligence officer. Mr Fhimah was born in Suk Giuma, Libya, on 4 April 1956. He speaks Arabic and English. 

According to the Federal Bureau of Investigation he has had three Libyan passports, and is said to have used three aliases. 

Mr Fhimah insists that he is a peace-loving, married father of five, who was "neither an intelligence man nor a politician". 

Abdel BasSet al-Megrahi 

A former director of Libya's Centre for Strategic Studies, Mr Megrahi was chief of airline security for Libyan Arab Airlines. He was in Malta in the lead up to the bombing and is also accused of being a Libyan intelligence officer. 

Born in Tripoli on 1 April 1952, Mr Megrahi learnt English in America where he studied in the Seventies. Mr Megrahi is married. The Federal Bureau of Investigation says he has at least four Libyan passports and has used up to nine aliases. In an interview with an ABC News reporter for US television in December 1991, Mr Megrahi said: "You judge me falsely, I'm a quiet man. I never had any problem with anybody. My life is clean."