It is more than 18 years since the bombing of Pan Am flight 103 killed 270 people in Lockerbie. Yet serious questions remain over who was behind the worst mass murder in British history.

The credibility of one of the most complex international terrorism investigations conducted by British police may hinge on the simple question of who bought a handful of clothes in a shop in Malta in December 1988 and whether the Christmas lights in the street outside were switched on at the time.

Last week a Scottish judicial body ruled that Abdelbaset al-Megrahi, who was sentenced to 27 years in a Scottish prison for his role in the attack, might have been wrongly convicted.

The trail of evidence that led to Megrahi’s conviction in 2001 seemed compelling enough at the time. Megrahi was a senior official in the foreign intelligence service of Libya, which then had a proven track record of sponsoring terrorist attacks against America and – through supplies of Semtex to the IRA – Britain.

At the time of the bombing in December 1988, Colonel Gad-affi, the Libyan dictator, was still smarting from the US bombing of Tripoli. The attack had killed his six-year-old niece and Gad-affi wanted revenge.

A CIA agent, planted inside Libyan intelligence headquarters, reported that the decision to attack an American aircraft had been made at a meeting chaired by Abdullah Sanussi, then deputy head of Libyan intelligence. Painstaking work by Scottish detectives and forensic analysis at the Royal Armament Research and Development Establishment (RARDE) in Kent had established that clothing found to have been wrapped around the device that blew up flight 103 had been bought in Malta.

German airport baggage records that had not been disclosed to the Scottish police until four months after the crash strongly suggested that the Sam-sonite suitcase used to contain the bomb had originated in Malta.

A baggage loading list suggested that an unaccompanied suitcase from a flight from Malta had been loaded onto the first leg of flight 103 at Frankfurt then transferred to the New York leg of the flight from London.

In Malta, detectives had traced one item of the clothing to a shop in the sleepy seaside resort of Sliema. There, a shopkeeper called Tony Gauci recalled how an Arab man had walked into his shop a few weeks before the bombing and bought a random selection of clothing.

These included a blue Baby-gro, a pair of checked trousers, an imitation Harris tweed jacket and a black umbrella. Interviewed by Scottish police nine months after the bombing, Gauci clearly recalled selling the Arab man other items.

Unknown to him, Gauci’s unprompted description of the man’s purchases seemed to exactly match the contents of the bomb suitcase as identified by RARDE. Gauci told police the man who had bought them was “Libyan”. He said he had seen him twice in Sliema after his unusual buying spree. He was confident he could identify him in a witness parade.

Gauci’s evidence was supported a few months later by the dramatic discovery of a tiny piece of circuit board embedded in the remains of a suitcase found in the hills around Lockerbie. The circuit board was part of an electronic timing device manufactured by Mebo, a company in Switzerland. The company later accepted that it had sold a batch of these civilian timers to Libya.

Separately, a CIA “supergrass” inside Libya named Megrahi and Al-Amin Khalifah Fhimah, another Libyan, as the two agents involved in the attacks. The supergrass – codenamed “Puzzlepiece” – claimed he had discussed the bomb plot with Megrahi and saw both men with explosives and the suitcase used for the bomb.

In November 1991 the Americans and British jointly accused the pair of the Lockerbie bombing. At their 2001 trial before three Scottish judges in the Neth-erlands, Fhimah was acquitted. But Megrahi was found guilty of the murder of 270 people. Gauci’s identification evidence was the linchpin of their verdict.

However last week, after a three-year investigation, the Scottish Criminal Cases Review Commission announced that it was referring his case to the Scottish Court of Appeal.

It dismissed claims by lawyers for Megrahi that vital evidence, including the circuit board from the Mebo timer, had been planted among the debris by police. Some of the more excitable conspiracy theorists suggested that was part of a plot by the security services to implicate Libya and exonerate Iran and Syria at a time when their neutrality was required in the run-up to the first Iraq war.

But, crucially, the commission did say it had identified six grounds where it believed a miscarriage of justice “may have occurred”. While the commission has inexplicably refrained from publishing details of each of these grounds, it is clear that doubts about Gauci’s testimony form the core of its concerns.

In his evidence Gauci had said Megrahi had looked “a lot” like the man to whom he had sold the bomb clothes.

But the commission found that four days before he identified Megrahi in an ID parade, he had been shown a photograph of him in a magazine article about the Lockerbie bombing.

That could be fatal to the prosecution case. Kirsty Brimelow, a defence barrister, said: “Showing a specific photograph of the suspect to the witness completely undermines the integrity of the identification process. There is a real danger that any subsequent identification would be flawed.”

Equally worrying was the precise date Gauci claims that Megrahi bought the clothes. The commission found that while Megrahi was in Malta several times in the weeks before Lockerbie, he would have been able to visit Gauci’s shop only on December 7, 1988.

Gauci always insisted the Christmas lights in the street outside were switched off when his Arab customer bought the clothes. But new evidence uncovered by the commission suggested the Christmas lights were in fact on on December 7. If true, that would conclusively rule out Megrahi as a suspect.

But if Megrahi didn’t carry out Lockerbie, who did? In the aftermath, it was widely believed an Iranian-based terrorist group, the Popular Front for the Liberation of Palestine-General Command (PFLP-GC), was responsible. The group, an offshoot of the Palestine Liberation Organisation, was said to have been acting on behalf of the Iranians, who wanted revenge for the accidental shooting down of an Iranian Airbus by the American war-ship USS Vincennes in the Gulf in July 1988.

Intelligence suggested $10m had been transferred by the Iranians into a PFLP-GC bank account on the eve of the attack. Ahmed Gibril, the group’s leader, was reported by a CIA informant to have celebrated the Lockerbie attack with a glass of champagne.

The group had a track record of aircraft bombings. One of its cells had been operating in Germany and had been rounded up by police just a few weeks before the Lockerbie attack. A Toshiba radio bomb, packed with Semtex and similar to that used to blow up flight 103, had been recovered during police raids. But several members of the cell had escaped. They were on the run with a second Toshiba bomb.

A key suspect in the early months of the police inquiry was Mohammed Abu Talb, a Palestinian terrorist who had been convicted of minor bombings in Scandinavia.

The evidence implicating Talb in the Lockerbie attack is best described as “strongly circumstantial”. He had links with some of the PFLP-GC gang in Germany and was in Malta buying clothes in early December 1998. At one point Gauci is said to have identified him from a photograph as the buyer of the clothes. When police raided his home in Sweden, they found clothes that had been bought in Malta. Some reports suggest some of these clothes could be traced to Gauci’s shop. A police phone tap revealed that before his home was raided one of his associates was told: “Get rid of the clothes.”

A calendar in his home had a ring around 21 December, 1988 – the date of the Lockerbie bombing. One of his associates said he had shaved his head at the time of the attack – a habit of Islamic extremists as they prepare themselves for attacks.

But Talb always denied knowledge of the Lockerbie attack. His possible complicity was quickly forgotten once Gauci appeared to positively identify Megrahi.

Now the Scottish commission has resurrected the possibility that Gauci may have been mistaken and that Talb – or some other, unidentified person – may have been his customer.

The appeal is due to be heard within a year. The five Scottish judges may decide to reject it. But if they uphold it, their decision will mark the single biggest miscarriage of justice in British criminal history.

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