The killing of 270 people on 21 December 1988, when a bomb destroyed the New York-bound Pan Am Flight 103 over the Scottish town of Lockerbie, was Britain's worst mass murder. It resulted in the conviction in 2001 of Abdelbaset Ali al-Megrahi - which to many who have studied the case is the country's worst miscarriage of justice. Yet, for the wider public, the only cause for concern is that Megrahi was allowed to go home to Libya. In the two and a half years since his plane took off from Glasgow Airport, David Cameron and Scotland's opposition parties have missed no opportunity to sling mud at the Scottish justice minister, Kenny MacAskill of the SNP, for his decision to grant the terminally ill prisoner compassionate release.

Everyone recalls Megrahi being greeted by saltire-waving supporters in Tripoli, but few know that, two years earlier, the Scottish Criminal Cases Review Commission (SCCRC - the statutory body that examines alleged miscarriages of justice) referred his case back to the Court of Appeal on six grounds, including that the trial court's judgment was unreasonable.

In convicting Megrahi, three Scottish law lords accepted the Crown's case that, on 7 December 1988, he went to a small shop in Malta and bought clothes, which were later placed inside the same suitcase as the bomb. At first glance, it appeared to fit: Megrahi was in Malta that day and the shopkeeper Tony Gauci said that he looked similar to a man who had bought clothes a few weeks before the bombing. But Gauci was certain that it was raining as the man left the shop; weather records showed that there had been no rain that day - Megrahi's only window of opportunity. Gauci also described a man taller and older than Megrahi.

The SCCRC discovered that the Crown had failed to disclose evidence to Megrahi's defence team, including documents proving that, before he first picked out Megrahi from a photo line-up in 1991, Gauci knew that a large reward was on offer from the US government and had "expressed an interest in receiving money".

Saving face
Why did Megrahi abandon his appeal? His explanation, revealed for the first time last month, is that MacAskill privately told a Libyan government minister that it would be easier for him to grant compassionate release if he dropped the appeal. MacAskill denies this, claiming that his SNP government had nothing to fear from the appeal.

Others disagree, among them Jim Swire, who lost his daughter Flora in the bombing and believes that Megrahi was wrongly convicted. They note that the Libyan's decision averted possibly the biggest crisis that Scotland's criminal justice system, the country's foremost independent institution, has ever faced. A different outcome would have caused acute discomfort to a party that stands for full independence.

Alex Salmond and his justice minister have stuck faithfully to the Crown Office line that nothing is amiss with Megrahi's conviction. The longer they maintain this fiction, the greater the scandal they are creating for themselves. Sooner or later, they will have to explain why they ignored all the warnings that their cherished system had got things very wrong. They would do well to start working on their explanations now.

John Ashton's "Megrahi: You Are My Jury - the Lockerbie Evidence" is published by Birlinn (£14.99)