He should never have reached this milestone. Abdelbaset al-Megrahi, stricken with terminal cancer, was given less than three months to live when he was spirited out of Greenock Prison and home to a delirious reception in Libya. But this week, the Lockerbie bomber, with his wife and five children, will mark the first anniversary of the Scottish Executive's decision to let him go, on "compassionate grounds".
A committee of American senators, furious at the refusal of any British politicians to appear before its inquiry into the release, is now appealing to "concerned citizens" to blow the whistle on any suspicious behaviour uncovered during the episode. The Senate Committee on Foreign Relations will seek people with "unique insight" into issues including Megrahi's medical condition and negotiations between BP and the Libyan government.
The appeal demonstrates the exasperation of the senators, in their quest to tease out more details of what they suspect was a "rigged decision" to get Megrahi out. But it also emphasises that, like the man convicted for the Lockerbie bombing himself, there are enduring mysteries over Megrahi that refuse to go away.
So who's idea was it to let him go, then?
The Scottish Executive originally opposed Megrahi's release and insisted that he be excluded from a prisoner transfer agreement (PTA), finally agreed with Libya in 2007. But, after Megrahi's condition was confirmed the following year, it started the process that would lead to his release.
The Justice Minister, Kenny MacAskill, maintains that the final decision was based on the advice of doctors. However, internal documents relating to the release record that a PTA application submitted by the Libyan government on Megrahi's behalf only three months before the release was "not structured to focus on health matters".
The Executive also denies making an agreement with the prisoner to persuade him to drop a potentially embarrassing appeal against his conviction, so his release could at least be speeded up. Megrahi's lawyers announced they had applied to withdraw the appeal on 14 August 2009, and six days later he was freed.
Although the then British government voiced its opposition to the MacAskill decision and stressed it was a matter for the Executive, UK ministers were not totally opposed to Megrahi's release. In a letter to the Executive, the then justice secretary, Jack Straw, reversed his opposition to Megrahi being included in the PTA, citing "overwhelming interests of the UK" – although he did not explain what these were. A Libyan politician, Abdulati Alobidi, also insisted to Scottish officials that he had been told Gordon Brown did not believe Megrahi should die in prison.
But he had three months to live: why is he still alive?
Medical advice presented to Mr MacAskill charted Megrahi's rapid deterioration and stated that his Libyan doctor came up with a prognosis of three months, and this was "confirmed by HMP Greenock's health centre doctor". The records also show that, in June and July 2009, several specialists agreed his cancer was "hormone resistant" – and said his prognosis had moved "to the lower end of expectations from 10 months ago". However, it added: "Whether or not prognosis is more or less than three months, no specialist would be willing to say."
Last month, the cancer specialist Karol Sikora suggested Megrahi could live for up to 20 years, although he later insisted he had said such a lifespan would be unusual. Last week, The Wall Street Journal reported that the specialists who treated Megrahi had not been consulted by the non-specialist who certified his condition as terminal. It was also claimed that the prisoner had disclosed his plan to begin chemotherapy at a doctor's suggestion – which should have alerted medics to the possibility that he was not three months from death.
The confusion over Megrahi's true condition has become a central issue for the Senate committee, which is now calling on Scotland to surrender Megrahi's medical records.
What was Tony Blair's role?
The former prime minister was deeply involved in the negotiations that ultimately paved the way for Megrahi's release. When the Scottish First Minister, Alex Salmond, was recently badgered by the US committee, he urged them to turn their attention to Mr Blair and "the deal in the desert" in 2007, when he is alleged to have agreed the PTA with Colonel Gaddafi as part of a wider plan to return Libya to the international community.
However, as The Independent on Sunday revealed last year, the process began four years earlier with a secret meeting between British, American and Libyan spies at the Travellers Club in Pall Mall, London. Immediately afterwards, Mr Blair and Colonel Gaddafi spoke on the phone and Libya was on its way back in from the cold. A former Blair adviser said this month that the PTA was a "reward" for Libya having given up its nuclear weapons. It has not been established if Megrahi's freedom was the price for closer co-operation.
What about BP?
BP, the central target of the US investigation, has acknowledged it had expressed concern to the British government about the progress of the PTA, but denied raising the Megrahi case. Mr Salmond said the company did not approach his administration over the issue.
But BP is inextricably linked to the Libya operation; in fact, it signed a £540m exploration deal during Mr Blair's visit to Tripoli in 2007, when the memorandum of understanding paving the way for the PTA was agreed. Sir Mark Allen, in charge of the Middle East and Africa department at MI6 until he became an adviser to BP in 2004, was deeply involved in the negotiations.
Were the Americans involved?
In public, the US has furiously criticised the Megrahi release, but behind the scenes it revealed a softer stance. Last year, the IoS revealed that the Americans had indicated that they would bankroll a scheme to put Megrahi under house arrest in Scotland, rather than see him go back to Libya. A letter leaked last week showed that they had secretly advised Scottish ministers it would be "far preferable" to free him than jail him in Libya. The US government refused permission for its official submission to the early-release process to be made public.
So, did he do it, or not?
Megrahi was found guilty of planting a bomb on Pan Am Flight 103 and sentenced to life in January 2001. The judges found that "there is nothing in the evidence which leaves us with any reasonable doubt as to [Megrahi's] guilt". His co-accused was acquitted. But several people have questioned his guilt, claiming the court ignored reports that Iran-backed Palestinian groups could have committed the bombing, and pointing out that the Pan Am baggage area at Heathrow had been broken into the night before flight 103 arrived. Tony Gauci, a Maltese shopkeeper, said he remembered selling clothing to Megrahi that was wrapped round the bomb. Mr Gauci was later reported to have been paid $4m by the FBI as a reward for his testimony. Megrahi has never admitted carrying out the bombing and had been planning a second appeal.Reuse content