It is three weeks since news emerged of the decision to allow the Lockerbie bomber to return to Libya to die, and the sense of unease is growing. Polls show that two-thirds of people in Britain, and a similar proportion in Scotland, where the decision was made, think the release of Abdelbaset Ali Mohmed al-Megrahi on compassionate grounds was wrong.

Legal and quasi-judicial decisions often spark outrage. Kenny MacAskill, Scotland’s justice secretary, had to defend his decision last week in the Scottish parliament. What has increased the sense of unease is the strong suspicion that the release was the direct result of deals done in the desert between Tony Blair and Colonel Muammar Gadaffi, the Libyan leader, deals subsequently refined by British government ministers.

Today we report on a letter written by Jack Straw, Britain’s justice secretary, to his Scottish counterpart in December 2007. In it he overturned a previous understanding that Mr Megrahi was exempt from a prisoner transfer programme agreed between Britain and Libya as part of the Blair-Gadaffi discussions. A few months earlier the government had been clear on that exemption. Lord Falconer, then lord chancellor, wrote to Alex Salmond, the leader of the Scottish National party, saying Libya had agreed that the Lockerbie bomber would serve out his sentence in Scotland.

What changed? The strong circumstantial evidence is that a lucrative agreement to allow BP to explore for oil off the Libyan coast was being held up by Mr Megrahi’s exemption from the prisoner transfer programme. The idea that the Westminster government had no view and no influence is not credible.

Last week we reported on a letter sent by Ivan Lewis, a Foreign Office minister, to the Scottish government. In it he said there was no legal barrier to the release of Mr Megrahi, adding: “I hope on this basis you will now feel able to consider the Libyan application in accordance with the provisions of the prisoner transfer agreement.” The Foreign Office said this letter did not imply the government was encouraging the release. It would be difficult, however, to find a form of language that provided much more encouragement.

Intelligent members of the public think they know what happened. They think the Scottish government would never have released Mr Megrahi without Westminster’s encouragement. They believe that encouragement was part of a deal to further Britain’s commercial interests in the country. That is why Gordon Brown has been so coy about condemning the release, while allowing himself the costless luxury of expressing revulsion at Libya’s celebrations of the bomber’s return home.

Diplomacy is murky. Deals done behind the scenes are not intended for public view. In this case, however, it does not seem murky at all, but perfectly clear.

It may be, of course, that this interpretation of events is wrong and the government’s claim that it had no part in and no influence on the decision can be backed up. It could be that ministers are as shocked and embarrassed by Mr Megrahi’s release as the voters.

That is not how it looks. It looks as if the feelings of the families and friends of the 270 victims of Lockerbie were sacrificed in the interests of keeping Colonel Gadaffi happy and getting access to his country’s lucrative energy reserves. If the facts are different, the government has an interest in releasing its records relating to the release. If not, suspicions will increase that a dirty little deal was done. The evidence we have unearthed points that way.