I am always surprised when people refer to Abdelbaset Ali al-Megrahi as the Lockerbie bomber. Even if he is guilty – something which, personally, I do not believe – he would only be a Lockerbie bomber, just one of many people who carried out a crime which would have taken a large network of people and lots of money to carry out. It amazes me that the British and American governments act as if the investigation into the bombing is somehow complete.

But I welcome the release of Megrahi, because I firmly believe that he is innocent of the charges made against him. Believe me, if I thought he was guilty I would not be pleased to see him released from jail.

His decision to drop his appeal, however, is deeply suspicious – I believe Megrahi made that decision under duress. Under Scottish law he did not need to abandon his appeal in order to be released on compassionate grounds. So why did he do it? It makes no sense that he would suddenly let it go.

In my time as the UN's observer at Megrahi's trial, I watched a case unfold that was based on circumstantial evidence. The indictment against him and al-Amin Khalifa Fhimah went to great lengths to explain how they supposedly planted a bomb on Flight 103, and yet Fhimah was acquitted of all the charges against him. It made no sense that Megrahi was guilty when Fhimah was acquitted.

The prosecution produced key witnesses that lacked credibility or had incentives to bear false witness against Megrahi. Tony Gauci, the Maltese shopkeeper who supposedly sold him the clothes that went around the bomb, had been fêted by the Scottish police who took him fishing. The Americans paid him cash following his testimony. The weakness of that testimony would have been a key component of Megrahi's appeal.

We will probably never really know who caused the Lockerbie bombing. So much key information was withheld from the trial. A luggage storage room used by Pan Am at Heathrow was broken into on the night of the bombing, and yet this information was withheld. The British have yet satisfactorily to explain why.

I want to know when the bomb was placed on the plane and by whom. We have to look more closely into the "London theory" – that the bomb was placed on the plane at Heathrow and not in Malta.

It would be childish to be satisfied with the conviction of just one person for a crime that clearly involved a large number of people. I find it very difficult to understand why there seems to be so little pressure from the British and American public on their governments to investigate the bombing properly.

The UK regularly talks of the need to pursue all terrorist atrocities. Yet how can the Government assure the public they really believe that, when they have virtually abandoned their investigation into the worst terrorist attack in the country's history?

We have to know what happened and the only way is a full public inquiry, either mandated by the House of Commons or by an investigative commission voted for by the UN's General Assembly. Time is of the essence. This crime is already 21 years old. To find out the truth we must act now.