In the public eye, the worst terrorist atrocity ever carried out in Britain, the murder of 270 people, remains a crime unsolved. Its perpetrators are unpunished.

The one man convicted of the bombing of Pan Am flight 103 over Lockerbie in 1988 was last week released from a Scottish jail. Abdelbaset al-Megrahi was given a hero's welcome in Libya, where he is now free to die from the cancer that was the ostensible reason for his release on compassionate grounds.

That the regime of Muammar Gaddafi would seek to make political theatre out of Megrahi's homecoming was inevitable. That is one of the Libyan regime's preferred styles of diplomacy. The implied message is that the ailing bomber was a hostage of the west, freed by some ingenious stratagem of the country's "brotherly leader".

Colonel Gaddafi's son, Saif, has stoked speculation that Megrahi's freedom was a quid pro quo for some concession in access to Libya's oil and natural gas reserves. That is denied by both the British government and by the devolved Scottish administration, which jealously guards its constitutional autonomy in judicial affairs such as prisoner release.

The Scottish justice secretary, Kenny MacAskill, engaged in a certain theatricality of his own when delivering Megrahi's fate. The Libyan's prostate cancer, he said, amounted to a death sentence in "a higher court". So the courts of men should show mercy. Thus, too, would Scotland as a whole, Mr MacAskill suggested, show its moral superiority to the merciless criminal.

It was a fine oration that summarised a sound liberal principle: that justice is not always the same as vengeance. But that is beside the point, because justice has never been served in relation to Lockerbie.

Megrahi did not testify in court, but there was enough incriminating testimony by others for a panel of judges to find him guilty. A co-defendant was acquitted for want of evidence. A coherent narrative of how the plot was hatched and executed did not come out. It is impossible that Megrahi acted alone.

It is also implausible that he will have been released from Scotland without consultation at the highest level of British government. But it suits Alex Salmond's nationalist administration to show that it can take momentous decisions. It also suits the Labour administration in Westminster for the decision to be seen to be Scottish.

Megrahi's extradition from Libya in 2000 to stand trial was the product of careful diplomacy. So was the Libyan regime's payment of compensation to families of the Lockerbie victims in exchange for a lifting of trade sanctions. After the 11 September attacks in 2001, Colonel Gaddafi was cajoled, threatened and enticed away from pariah status in the world. He renounced weapons of mass destruction; he opened his country's energy sector to foreign investment.

Those deals are bound to colour perceptions of the decision to free a Libyan intelligence officer serving time in a British jail for a terrorist attack. They make the "compassionate release" story look naive, dishonest or both.

Meanwhile, the group whose interests are least served is the relatives of the victims. Their entitlement to know the truth about flight 103 has been forgotten by the British government. There is no one behind bars in connection with the most murderous terrorist attack on British soil. Justice has not been seen to be done. Instead, all that is in the public view is a sequence of events that are easily threaded into a narrative of conspiracy.

Perhaps that is not so. Maybe Megrahi was released because the Scottish administration took pity on him. In which case, the government in Westminster must have an opinion on whether that was the right decision. But if the prime minister has a view, he is not sharing it. That is a mistake. Official silence over the legacy of the Lockerbie bombing has gone on for far too long.

• This article was amended on Friday 28 August 2009. We said that Abdelbaset al-Megrahi was found guilty by a jury; in fact, he was found guilty by a panel of judges. This has been corrected.