Abdelbaset Ali al-Megrahi was released from prison in Scotland and returned to Libya last week.
He was convicted in 2001 of murdering 270 people when Pan Am Flight 103 exploded above the Scottish town of Lockerbie 13 years earlier.
Yet there is a huge amount of evidence to support Megrahi’s contention that he is innocent.
Jim Swire, whose daughter Flora was killed on Flight 103, has branded Megrahi’s conviction “one of the gravest miscarriages of justice in history”.
Evidence uncovered by the victims’ families, and the journalists John Ashton, Ian Ferguson and Paul Foot, has consistently shown that Megrahi was innocent and that he was framed to cover up what really happened.
We look at the key questions in the case.
What was the official version of events?
Megrahi was accused of conspiring with another Libyan man to place a bomb in a suitcase, which was loaded onto a flight from Malta to Frankfurt, where it was transferred onto a flight to Heathrow.
The bomb was built into a Toshiba radio-cassette player.
It was allegedly fitted with a timing device supplied to the Libyan intelligence service by a Swiss company. The company shared premises with a Libyan company, with which Megrahi had links.
Megrahi was also alleged to have bought the clothes in the bomb suitcase from a shop in Malta in December 1988.
This version of events does not hold up under scrutiny.
Was initial evidence suppressed?
Disinformation came to the fore within hours of the attack and vital evidence was suppressed.
Police officers and volunteer searchers spoke of US agents removing items from the crash site.
Why were key witnesses paid by the CIA?
The prosecution’s star witness in the case, Abdul Majid Giaka, was exposed during the court hearing to be a money-motivated fantasist in the pay of the CIA.
In the end the court dismissed his evidence.
Another key witness, shopkeeper Tony Gauci, testified that Megrahi bought clothes in Malta that were found with the bomb. Gauci gave contradictory evidence.
At one point he suggested that the man who bought the clothes was older and taller than Megrahi, and that the purchase took place at a time when Megrahi had an alibi.
Gauci picked out Megrahi in a line-up, but only after being shown a photograph of the suspect four days earlier.
Gauci was paid £1.2 million by the US for his evidence and many of his conflicting statements were withheld from the defence.
Megrahi’s co-accused, Lamin Khalifah Fhimah, was acquitted, undermining the charge that the men conspired together to bomb the plane.
Who found the key evidence?
The central plank of the case against Megrahi was the correlation between the bomb timers that had allegedly been sold to the Libyan authorities and a tiny fragment of circuit board found near Lockerbie.
But who found the circuit board and when?
At one point an unnamed former senior Scottish police officer said that he had planted the fragment at the crash site by order of the CIA.
Another version of events states that the evidence was found by an unnamed Scottish worker in a field outside Lockerbie “on a misty morning in early April” 1989.
Yet another says it was found “sometime in 1990” in a “piece of charred shirt” by the FBI’s forensic expert Thomas Thurman.
The assistant director of the FBI forensic laboratory says the British authorities found the fragment a whole year before Thurman got it.
And who identified the fragment as part of the timer?
Some say Thomas Thurman, others a “veteran CIA analyst”, and yet others say the military forensic scientist Dr Thomas Hayes – who was at the heart of the investigation that wrongly jailed the Maguire Seven in the 1970s.
So the key link with the Libyans – and so with Megrahi – was made in April 1989, or in June, August, October or November 1990 depending on your preference.
How did the bomb get on the plane?
Experts have repeatedly disputed the official account that the bomb was put on a feeder flight at Malta and was transferred twice – at Frankfurt and Heathrow airports.
Some people believe the bomb was put on the plane at Heathrow.
Megrahi’s first appeal case in 2002 heard evidence from a security guard at Heathrow who reported a break-in close to the shed where, later the same day, the baggage for the Pan Am plane was handled.
And the chief baggage handler at Heathrow, John Bedford, testified that he saw two additional suitcases had been loaded into the relevant container for Flight 103.
The explosion occurred precisely where those cases had been placed, above a single layer of baggage that Bedford had already packed into the container.
There is another more shocking possibility as to how the bomb came aboard.
Elements within the CIA were at this time allowing Middle Eastern dealers to ship drugs to the US in return for help in releasing US hostages.
Some sources, including Pan Am’s own insurance investigation, believe that this happened on Flight 103.
If so, the luggage with the drugs would have been protected by US intelligence. Normal security restrictions and checks on baggage would have been removed.
This is supported by the fact that two large quantities of what appeared to be heroin were found – one on a Lockerbie golf course and the other in a suitcase discovered by a farmer a couple of miles to the east. These then disappeared.
There was a party of US Defence Intelligence Agency (DIA) officers on the Pan Am flight, led by Major Charles McKee, returning from a failed hostage-rescue mission in Lebanon.
It is claimed that McKee was planning to denounce the drug running scam on his return to Washington.
What sort of bomb was used?
The official account of the attack involves a bomb with a conventional timer making a convoluted journey through airports in Malta, Frankfurt and London, set to detonate 38 minutes into the third flight.
Some commentators have suggested that a more likely possibility is that the bomb had an air-pressure switch – which would detonate between 32 and 42 minutes into a flight.
The evidence for this comes from the initial investigation into the bombing.
The line of the British and US police and intelligence services directly after the attack was that the bombing was carried out not by Libyans, but by a group based in Syria.
This group had been hired by Iran to avenge the shooting down of an Iranian civil airliner by a US warship in the summer of 1988.
In the months before the bombing a group associated with the Popular Front for the Liberation of Palestine-General Command were arrested in Frankfurt, Germany.
The leader of this group was Ahmed Jibril, who had the protection of the Syrian government.
Investigators were in no doubt that the bombs found in the Frankfurt raids were designed to blow up aircraft.
They were fitted with air-pressure switches, packed inside Toshiba radio cassette recorders and at least one of them was never recovered.
The man convicted of making these bombs, Marwan Khreesat, has been given immunity from prosecution for the Lockerbie bombing.
What role did Western foreign policy play?
Immediately following the Lockerbie bombing, the charges against this group fitted with US and British foreign policy.
Both Western powers had broken off relations with Syria. The war between Iraq and Iran ended in the summer of 1988 with the US and British governments firmly on the side of Iraq.
But this political alignment soon changed. In April 1989 George Bush senior rang Margaret Thatcher and encouraged her not to proceed with the inquiry into the Syrian-Iranian connection.
A year later, British and US armed forces prepared for an attack on Saddam Hussein’s occupying forces in Kuwait.
The coalition desperately needed troops from an Arab country and for Iran to remain neutral. The troops were conveniently supplied by Syria.
As the political allegiances in the region changed, so did the official investigations into the Lockerbie disaster.
In November 1991 the US and British governments announced that Megrahi and Fhimah were charged with planting the bomb which brought down Pan Am 103.
Libya gave up Megrahi for trial after a number of years – on condition of the lifting of sanctions, which had cost Libya an estimated $30 billion.
As the West’s desire to bring Libya back into the diplomatic fold and Western oil companies’ desire to make cash in the country have converged, there has been a further realignment of relations to ensure good trade.
That is the context in which Megrahi has been released on “compassionate grounds”.
Was there a cover-up?
British relatives of the victims of Lockerbie, including Martin Cadman, met the US President’s Commission on Aviation Security and Terrorism in 1990.
“After we’d had our say,” says Cadman, “the meeting broke up, and we moved towards the door. As we got there, I found myself talking to two members of the commission – I think they were Senators.
“One of them said, ‘Your government and our government knows what happened at Lockerbie. But they are not going to tell you.’”