I became involved with Abdelbaset Ali al-Megrahi quite by accident. Like many people I had been suffering from Lockerbie fatigue. For me, and for you, I suppose, life had moved on from that horrendous crime over 20 years ago and the imprisonment of the Libyan murderer. That was that.

At least it was, until I agreed, by chance, to sponsor the showing of a Dutch documentary about the Lockerbie bombing at Parliament. I invited all MSPs and researchers, and indeed the press corps, to see this film. One MSP and one member of the press came, and I really only saw it because I felt obliged to attend. But that film changed my perspective. From that casual moment, and from much that I have learned since, I am convinced not only that Megrahi was not found guilty "beyond reasonable doubt", the test in Scot's law, but that he is an innocent man.

He is not a saint, of course – he had a history with Libyan intelligence – but his hands are clean over Lockerbie. For you should recall that five months before the bombing of Pan Am flight 103 on that dark, wild December night just before Christmas in 1988, an American military cruiser, the Vincennes, shot down an Iranian passenger plane carrying 290 pilgrims. No one has been charged, let alone prosecuted, over that, even though it was all captured on film.

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It is reasonable to deduce that when an American plane carrying, as some believed, military personnel back home to their religious festival, is blasted out of the sky, the finger of suspicion should not point first at Libya. Iran, maybe. However, Iran had to be kept on side because of the Iraq/Kuwait conflict. The Iran/Syria connection was soon dropped, and so Libya was indeed blamed. Here was a credible culprit.

To successfully frame a nation, pick one like Libya, in which all the baddies of the Middle East are personified in a recognisable hate figure like Gadhafi. If you want to frame a man, pick one with a feasible track record. Then first sell it to the world through the press and, hey presto!

But back to that film, which has not yet been seen here. After watching this disconcerting documentary, which challenged the reliability of key evidence, I got into conversations that night with Dr Jim Swire, with a forensic police scientist who had to label those bodies scattered across hundreds of acres of dark wintry hillside, with Father Patrick Keegan, the priest who lived in Sherwood Crescent (the only person who survived in that street) and with others. None of them supported the case against al-Megrahi.

Since then I have met the man at the centre of it all on several occasions. Our first meeting took place on a blustery morning some months ago. Afterwards I was confronted by a crowd of reporters who waited until I emerged one hour later from speaking to a man so detested, so reviled by many that death in prison from cancer would be too good for him.

He was sitting in front of a laptop, across the table in a room set aside of lawyers and their clients. His English was excellent and I remember trying to impress upon him that I was there for the duration, and not just this one visit. I told him that if I thought for one minute he was guilty I would walk out of the room. But he was intent on scrolling through the pages of the trial, pausing now and then to emphasise a point. Perhaps he was listening.

On subsequent visits we could go straight to the point, and deal with "prisoner transfer": to qualify he would have to abandon the appeal which could allow him to clear his name. We also talked more of his family and the growing need, as his health worsened, as it clearly was doing, to be with them. It was then that his composure was momentarily lost; the emotion and tension were tangible. But although his priority was to be with them in his last days, he told me he did not want his name to go down in history as the Lockerbie bomber. He told me, in short, that he did not do it. I told him again that I thought he was innocent.

Let me make one thing clear: I understand the hatred some feel for him, particularly the US relatives of the dead. It is, however, misplaced and it is in order to unravel for them the true story of Lockerbie, as much as to liberate an innocent man, that I and others worked hard for his compassionate release. This would have allowed the appeal process to be exhausted and evidence-led. The Scottish Criminal Cases Review Commission considered there was evidence vital enough for it to consider that there might have been a miscarriage of justice. That evidence, particularly relating to the identification of al-Megrahi linking him directly to the bombing has never and will now never be tested in a Scottish court.

My final meeting with him was on 23 July. He requested that it be private and I have kept my word till now. Apart from discussing his deteriorating health, increasing frailty and his family, we discussed at length his compassionate release. He wanted my advice. I told him I thought he had nothing to lose because if it was rejected he could abandon his appeal and take the prisoner transfer route. I advised him to consult his legal team.

The next day he applied for compassionate release. Stupidly, I thought there was a good chance that after his death at home his appeal could still be pursued, by his family. But, like al-Megrahi, I am a tiny cog in an elaborate mechanism. Last week he abandoned his appeal. His counsel advised the court that he believed that to do so would "assist" with his "applications".

The previous week I had received an email from a whistleblower in the Justice Department telling me that the Libyan officials were being told in no uncertain terms that he must drop his appeal or there would be no compassionate release.

Al-Megrahi was a desperate man, but I believe there are other desperate men and women – in the US Justice Department and in Whitehall, – all with their own reasons for wanting that appeal to be ditched. Now he is home, but he is still, officially, a guilty man.

Those who believe him guilty are crying foul. So are those of us who believe him innocent. And then there are those who are happily sipping their claret, their eyes on a comfortable unblemished retirement. As for any inquiry, that's out there in the long grass. They are people in authority who are relying on Lockerbie fatigue setting in again. It mustn't.

The SNP's Christine Grahame is a Member of the Scottish Parliament