Leading article: Lockerbie's unanswered questions

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It was the worst air disaster in British history. Pam Am Flight 103 broke apart at 31,000ft above Scotland, just one hour into its voyage from London Heathrow to New York's JFK airport on 21 December 1988. Scattering wreckage over a 10-mile area, the burning jet finally crashed into the small town of Lockerbie, destroying houses, a petrol station, and setting fires that reached 300ft into the sky. Eleven residents together with all 259 passengers and crew lost their lives that night.

Tomorrow marks the 20th anniversary of the tragedy. In the intervening period, the Libyan government has been blamed for the bombing; the families of the victims have received a $1.5bn compensation package authorised by Colonel Gaddafi, and Pan American airlines has filed for bankruptcy. In 2001 a panel of Scottish judges, sitting in the Netherlands under special arrangements agreed with the Libyan government, convicted Abdelbaset Ali Mohmed al-Megrahi, a Libyan intelligence officer, of 270 counts of murder for his part in the bombing. The narrative is far from complete, however. That al-Megrahi's case took 10 years to reach trial is remarkable; that serious questions still remain about the credibility of the evidence used to convict al-Megrahi is a scandal.

In June 2007, the Libyan's defence team was granted leave by the Scottish Criminal Cases Review Commission to appeal, for a second time, his conviction. The commission gave six grounds for believing a "miscarriage of justice may have occurred", chief among them that the evidence given by the prosecution's witness Tony Gauci, who identified al-Megrahi, was flawed.

Since then, his defence team has revealed that it was denied access to papers from a foreign government that were made available to Scottish police, but not defence lawyers. It also alleges that Gauci was offered a $2m reward in return for giving evidence. The substance of the claims will be measured at an appeal which begins next spring – providing al-Megrahi, who has been diagnosed with prostate cancer, is still alive.

In any event, the outcome could not be more important. Twenty years after that fatal flight, there remains a very real possibility that justice has still not been done.