Against the festive backdrop of this weekend's preparations for Christmas, services are being held on both sides of the Atlantic to mark a grim anniversary. Twenty years ago today, death rained down from a wintry sky on to a small Scottish town, forever tying the name Lockerbie to the worst terrorist atrocity on British soil and the costliest attack in terms of American lives before September 11, 2001. Just after 7pm on December 21, 1988, Pan Am Flight 103 from Heathrow crossed into Scottish airspace and was given clearance to begin its transatlantic crossing to New York's John F. Kennedy Airport. Three minutes later, a chain of events stretching back through London, Frankfurt and Malta ended in a bomb blast that brought the Boeing 747 down and claimed the lives of all 259 people on board and 11 people on the ground. Most of those who died were Americans returning home for Christmas. Their families formed close ties with the relatives of the 41 British victims and the people of Lockerbie. The dead will be commemorated at services in Lockerbie, at Arlington National Cemetery outside Washington and at Syracuse University in New York, alma mater of 35 of the victims. But while they are united in their common grief, the American and British relatives have become divided in the run-up to the anniversary over whether justice was properly done for their loved ones. Abdel Basset Ali al-Megrahi, a Libyan intelligence agent, was convicted of the crime under Scots law at a specially convened trial in the Netherlands in 2001. Megrahi, 56, is serving a life sentence with a minimum non-parole period of 27 years. He has repeatedly protested his innocence and is awaiting the outcome of an appeal, which is scheduled to be held in the spring. A campaign for Megrahi's release - bolstered by a number of British public figures who claim he is the victim of a miscarriage of justice - has gained momentum because of his failing health. An application by Megrahi to be released on bail while his appeal is being considered was refused last month because of the gravity of the crime. In a statement, Megrahi said: 'I wish to reiterate that I've nothing whatsoever to do with the Lockerbie bombing and our fight for justice will continue whether I am alive to witness ... my name being cleared.' The chairman of the Victims of Pan Am Flight 103 support group, Glenn Johnson, shares the belief of many of the US victims' relatives that justice has been served with Megrahi's incarceration and the conviction should be left well alone. Mr Johnson, whose daughter Beth Ann was among the 180 Americans killed, told the Sunday Morning Post: 'So far we see nothing new, only allegations in the press that have already been investigated and proved false. We have faith in the Scottish courts.' Dan Cohen wrote a book about the case after his daughter, Theodora, died in the attack. He has little doubt as to who is to blame for his family's suffering. 'I am convinced that al-Megrahi is guilty, and I don't know a single American family who thinks he did not do it,' he said. 'I want him to die in a Scottish jail.' Megrahi's lawyer fears he may well die in jail. Speaking last week from his office near Glasgow, Tony Kelly said Megrahi had been diagnosed with prostate cancer, which had 'spread and is therefore at an advanced stage'. Mr Kelly said it was Megrahi's intention to remain in Scotland upon release to receive palliative care for his cancer and to stay with his family, who had relocated to Glasgow to be near him. Among those who question Megrahi's conviction is Jim Swire, the spokesman for the UK Families Flight 103 group, who lost his daughter Flora in the disaster. In a recent letter to The Herald newspaper, Dr Swire wrote: 'Does anyone suppose they would feel any lasting benefit if Megrahi was forced to die in prison, far from his family? 'Would such a fate advantage those still grieving after 20 years for the loss of loved ones at Lockerbie? I don't believe it would.' Robert Black QC, professor emeritus of Scots law at Edinburgh University, was the architect of the Lockerbie trial held on neutral territory at Camp Zeist in the Netherlands. He helped persuade Libya to hand over Megrahi to stand trial, but he shares Dr Swire's doubts about the verdict. 'The evidence does not support Megrahi's guilt,' he told the Post. 'The attitude of the [US] relatives is irrelevant to that legal issue. I fail to understand how having an innocent man in jail can be justified simply because some relatives want to see him there.' Megrahi came to be sitting in a prison cell in Greenock, near Glasgow, after the most complex and expensive criminal investigation in Scottish legal history. In the immediate aftermath of the attack, the finger of blame fell on a Palestinian terror group, the Popular Front for the Liberation of Palestine - General Command (PFLP-GC), after it was linked to a radio-cassette player that housed the bomb. Attention would later turn to Libya when fragments of the Swiss-made timer used to trigger the Lockerbie bomb were matched with a similar device seized from a Libyan intelligence officer in Senegal. A possible motive was thought to be revenge for US air strikes, launched from British airbases, against Libyan targets in the mid-1980s. Investigators concluded that the bomb was concealed in a Samsonite suitcase. Clothes packed in the case were recovered and traced to Malta, where a shopkeeper, Tony Gauci, identified Megrahi as the man who bought them. 'There were two findings that were absolutely crucial to the guilty verdict,' Professor Black said. 'Both were contrary to the weight of evidence.' The first was that Mr Gauci identified Megrahi, even though the best he could say was that Megrahi 'resembled a lot' the buyer of the clothing. The second was that the judges wrongly accepted that the clothes were bought on December 7, 1988, when Megrahi was known to be in Malta, and not on another possible date identified by Mr Gauci, November 23, when he was not. The Scottish Criminal Cases Review Commission recommended last year that his appeal against conviction proceed on the basis of a possible miscarriage of justice. In the course of the commission's three-year review, questions arose about a secret document that is understood to shed light on the origins of the timer used. Campaigners say the document, which was withheld from Megrahi's defence team, could clear his name. Megrahi's case was not helped by his former job as head of security for Libyan Arab Airlines. LAA had an office in Malta, which suggested he was in a position to carry out the bombing. His claims of innocence were further dented in 2002, when Tripoli agreed to pay about US$3 billion to the families of the 270 killed. Doubts remain, however, about whether the payout was a genuine admission of responsibility or merely a pragmatic move by Libyan leader Colonel Muammar Gaddafi to have UN sanctions lifted and Tripoli removed from Washington's list of state sponsors of terrorism. Provided he lives long enough, Megrahi could be a free man next spring, leaving the obvious question unanswered: If he did not bring down Pan Am Flight 103 on that cold night 20 years ago, who did? The case spawned a slew of theories but most return to the earliest suspect, the PFLP-GC, and the belief that it carried out the attack at Iran's behest in revenge for the accidental shooting down of an Iranian Airbus by the USS Vincennes in July 1988.