Ever since I sat on a zebra-striped sofa in the El Morocco nightclub in New York at three o'clock one afternoon and listened to a man telling me how America was governed by a group of faceless industrialists who met regularly in a Las Vegas bunker, I have shrunk from conspiracy theories.

They are the enemy of truth, and breed a kind of obsession that is a diversion from genuine investigation. So I react instinctively against the latest claims to emerge from the 20-year-old saga that surrounds the Lockerbie bombing that killed 270 people on a Pan Am flight in 1988.

They suggest, improbably, that US security services may have known about the attack before it happened - and cynically allowed it to go ahead to preserve their undercover contacts inside terrorist groups.

Since this falls into a kind of Bermuda Triangle of supposition piled on speculation, it might be thought that there was little point in pursuing it. Except that now a moral dimension has entered the story. In a Scottish jail, the sole Libyan convicted of the atrocity is dying. Abdul Baset Ali Mohmed al-Megrahi, 55, is said to have terminal cancer and has been given weeks to live. Pressure has grown on the Crown Office in Edinburgh to bring forward his appeal, due some time next year.

All of which gives the need to resolve this matter a greater urgency. Is it enough to allow the wheels of justice, which have revolved slowly and imperfectly over the past 20 years, to grind out an eventual decision, when the man at the heart of the case is dying?

You do not have to be a conspiracy theorist to recognise that nagging questions have gnawed away at the Lockerbie case since the first investigations began. The veteran campaigner, Tam Dalyell, who describes himself as a “professor of Lockerbie studies”, is convinced that neither al-Megrahi nor the Libyan Government had any involvement. He, along with the Rev John Mosey and Dr Jim Swire, who both lost daughters in the atrocity, believe that there has been a spectacular miscarriage of justice.

They have raised questions about basic evidence in the original case. They have challenged eyewitness accounts offered by the chief prosecution witness, the Maltese shopowner who originally identified Megrahi as a suspect. They have raised doubts about the forensic evidence, and have pointed out that al-Megrahi, a civilised and intelligent man, is a most unlikely terrorist.

Last weekend, their campaign was given fresh impetus when Robert Fisk, the veteran Middle East correspondent, reported that Abu Nidal, the Palestinian terrorist responsible for some of the worst attacks of the 1970s and 1980s, may have been working for the Americans before the invasion of Iraq. Secret documents - the very phrase is a conspiracy idiom - written by Saddam Hussein's security services state that he had been colluding with the Americans trying to find evidence linking Saddam and al-Qaeda. Abu Nidal's alleged suicide in 2002 may have been an execution by the Iraqis for his betrayal.

From this tenuous connection stems the idea that the US security services may have had previous contacts within Abu Nidal's terrorist organisation, the Popular Front for the Liberation of Palestine-General Command, which many experts have long believed was the real perpetrator of Lockerbie.

Mr Dalyell, who thinks there may be some weight to this theory, points to incidents such as notices that went up in the US Embassy in Moscow in the days before the bombing, warning diplomats not to travel on PanAm flights, and how senior South African figures were hauled off the plane before the flight, almost as if there had been advance warning.

For me, this kind of evidence strays into the territory of “the second gunman theory” that bedevilled the Kennedy assassination. But there is one aspect of the case that I have never understood: why was it that, for the first 18 months of the investigation, Scottish police, US investigators and European security agents were convinced that the perpetrators were Abu Nidal's PFLP? And why was it that, in the run-up to the Gulf War, when good relations with Syria and Iran were important to Western interests, attention switched abruptly from Abu Nidal's terrorists, and on to Libya?

These matters have never satisfactorily been explained, and in the interests of common justice they should be addressed. For the sake of the Flight 103 victims, for the wider interests of Western security, and for the man now dying in a Scottish prison, there is a need for a proper inquiry. It does not have to be as wideranging as the Warren Commission that examined the Kennedy case, but it does need to be international, and to have US backing. The appeal in Edinburgh next year will examine legal aspects of the case, but it cannot extend to the wider issues that demand resolution.

Just possibly a new president taking office next January will find in his in-tray persuasive evidence pointing to a reopening of the case. There are powerful moral reasons for dusting it off and asking a basic question: who was responsible for Britain's worst terrorist outrage?

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