The suggestion that Iran, not Libya, bombed Pan Am 103 adds another twi st to the tale of the Lockerbie disaster. John Arlidge explains
The Lockerbie bombing is the biggest unsolved murder in British criminal history. Some 270 people died at 7.03pm on Wednesday 21 December 1988 when a Pan Am clipper, Maid of the Seas, was destroyed by a bomb. No one has been convicted of the terro rist crime.

The investigation into the disaster has spawned as many theories as there were victims. Some say that Palestinian, Syrian and Libyan terrorists were responsible, others that it was a secret service plot. The targets of the attack have been variously described as a South African government delegation travelling to Washington, US intelligence officers returning from a mission to rescue Western hostages in Beirut, the American ambassador to Lebanon and drugs smugglers. More than six years after the first pieces of wreckage were recovered by search teams in the Scottish Borders, no convincing explanation has been produced.

Investigators have uncovered a handful of simple facts about the Lockerbie bombing which are uncontested. A brown Samsonite-type suitcase was loaded on to Pan Am Flight 103a at Frankfurt airport on 21 December. The suitcase contained an improvised explosive device concealed in a Toshiba radio cassette recorder. Around one pound of Semtex high explosive had been used. The Boeing 727 left Germany and travelled to Heathrow airport where the suitcase was transferred on to the jumbo jet operating Pan Am Flight 103 to New York. The bomb exploded when the Boeing 747 reached 31,000 ft. In 100mph head-winds, the aircraft, travelling at 500ft a second, was torn apart. The wreckage was found over a 200-square-mile area.

Conspiracy after conspiracy has been laid on top of these simple facts by prosecution authorities in Britain and America and by investigative journalists. Two main theories predominate:

The `Official' version

The charge: Colonel Gaddafi, the Libyan leader, authorised the attack on Flight 103 in retaliation for the US raids on Tripoli and Benghazi in April 1986. Two Libyan agents planned and executed the operation.

The evidence: A fragment of circuit board allegedly from the bomb's timing device was found by search teams at the crash site. Detailed forensic analysis revealed that it had come from a batch of timers made by a Swiss company, Mebo. .Detectives who interviewed Edwin Bollier, the firm's owner, discovered that the timer was one of a series that Mebo may have supplied exclusively to Libya.

The timer fragment was found in the scorched remains of clothing which, further detailed forensic work revealed, had come from a shop in Malta, called Mary's House. Detectives questioned Tony Gauci, the shop's owner, about the clothes. He told them that a Libyan had visited the shop in 1988 and purchased, seemingly at random, a selection of items - a man's Harris tweed jacket, a baby suit, an umbrella. After two years of questioning, Mr Gauci identified the Libyan as Abdel Basset Ali Al-Megrahi.

Mr Al-Megrahi worked in Malta for Libyan Arab Airlines. He regularly travelled through the island's Luqa airport where he worked with another Libyan, Lamen Khalifa Fhimah, Malta station manager for the Libyan carrier. A Libyan named Jaika, who defected to the US, told investigators that on 21 December Mr Al-Megrahi had placed an unaccompanied suitcase on board Air Malta flight KM180 at Luqa. It flew to Frankfurt. Baggage handing records at Frankfurt indicate that the suitcase was transferred to Pan Am flight 103a.

The Result: On 14 November 1991 at simultaneous press conferences at the Crown Office in Edinburgh and the US Justice Department in Washington, warrants were issued for the arrest of Mr Al-Megrahi and Mr Fhimah. The pair were jointly charged with conspiracy and murder. The indictments alleged that they had used their knowledge of the international airline business to circumvent security procedures and place the bomb on Flight 103. They have consistently denied the charges and even though UN sanctions have been imposed on Libya in an effort to force Colonel Gaddafi to surrender the suspects for trial, the pair remain at large.

From the day the warrants were issued, Lockerbie relatives, MPs, the suspects' lawyers and investigative journalists have questioned the allegations against Libya. Their investigations have revealed flaws in the judicial case, including mistakes in the indictments against the two men and weak and confused evidence. Many now suspect Libya was not directly involved. The attack on Flight 103 was, they say, simply Iranian revenge for the shooting down in July 1988 of an Iranian Airbus in the Gulf. The airliner, wth 290 people on board, was destroyed by the USS Vincennes after navy personnel mistook it for a jet fighter. This suspicion has given rise to ...

The alternative theory

The charge: Following the Vincennes incident, Iran ordered the Syrian-backed terrorist group, the Popular Front for the Liberation of Palestine - General Command, to avenge the deaths of its citizens by attacking an American airliner. Britain and Americaknew that Iran and Syria were behind the atrocity but covered up the evidence and cobbled together a case against Libya instead, to protect Western strategic interests in the Middle East following the Iraqi invasion of Kuwait.

The evidence: In the spring of 1988, Colonel Gaddafi withdrew funding for the PFLP-GC after its leader, Ahmed Jibril, refused to co-operate with other Middle East terrorist groups which Libya was funding. Within hours of the Vincennes attack, Mr Jibril, faced with the loss of his main sponsor, sent a message to Tehran. The message, which was intercepted by intelligence agencies, offered his condolences and expressed support for the Iranian government.

On 8 July three separate intelligence agencies spotted Mr Jibril's right-hand man, Hafez Hussein Dalkamoni, in Tehran. He was seen in the company of Ali Akbar Mohtashemi, the hardline Iranian interior minister who supervised Iranian funding of Middle East terrorist groups.

A month later the Mossad, Israel's secret service, warned the West German authorities that a terrorist group was planning attacks in the Frankfurt area. In September and October the BKA, Germany's criminal police, mounted a surveillance operation on a flat in Neuss, Frankfurt, where Mr Dalkamoni had been seen. On 27 October 1988 operation Autumn Leaves, as it had become known, ended when the BKA arrested 17 people in Neuss and three other German cities.

Officers were stunned when, after arresting Mr Dalkamoni and Marwan Khreesat, a known Jordanian bomb maker, they found a primed bomb inside a Toshiba radio-cassette recorder in the boot of their Ford saloon car. Six months later three further bombs were found at a flat in Frankfurt.

Despite the discovery of the Toshiba bombs, intense questioning of Dalkamoni and the other suspects failed to reveal any direct link with the Lockerbie bombing. To the dismay of Scottish police, a federal judge sitting in Frankfurt freed Mr Khreesat, saying there was no evidence that he had committed a criminal offence. Khreesat immediately fled to Jordan. None of the German suspects was charged in connection with Flight 103.

Despite this setback, Western intelligence agencies continued to suggest that Mr Jibril, acting on orders from Ali Akbar Mohtashemi had instructed Mr Dalkamoni and Mr Khreesat to carry out the bombing.

On 2 August 1990, Iraqi forces invaded Kuwait. Suddenly the rumours circulating in Washington and London about Iranian and PFLP-GC involvement in Lockerbie stopped. Within days of the invasion, James Baker, then US secretary of state, flew to Geneva where he met high-ranking Syrian officials. His visit culminated in a face-to-face meeting with Hafez al-Assad, the Syrian president. It is unclear what the implications of the ground-breaking meeting were. But many observers have speculated that because Syrian support and Iranian acquiescence was vital to the success of the Desert Storm operation to liberate Kuwait, the US, through its intelligence network, decided to re-focus the Lockerbie investigation on Libya.

When, in November 1991, President George Bush and Douglas Hurd, the Foreign Secretary, ordered Libya to hand over Mr Al-Megrahi and Mr Fhimah for trial in Scotland or the US, both men angrily dismissed suggestions that either Iran or Syria were involved in the terrorist attack. George Bush described allegations of Syrian involvement as "a bum rap".

The Result: The Foreign Office last night repeated its insistence that no proof for this theory has been produced. But in the past two years, fresh evidence has emerged which lends weight to the claim. In 1993 a former American intelligence officer, Lester Coleman, alleged that a covert drug smuggling operation from the Lebanon to the US, via Cyprus, Frankfurt and London, had been penetrated by terrorists. The operation was, he claims, used by US intelligence to help identify major drug dealers in the US. The German-based terrorists used their knowledge of the covert drugs operation to switch an unaccompanied bag containing the drugs, with an identical bag containing the bomb which destroyed flight 103. The allegation has never been proven. It remains that last unexplained mystery of the Lockerbie story.