All 259 passengers and crew were killed, and 11 people in the Scottish town of Lockerbie died when the aircraft's wings and fuel tanks plunged to the ground. There were 189 Americans on board the deadly flight.
In 2001, Al Megrahi was found guilty of carrying out the bombing and sentenced to life in jail by a Scottish court sitting in the Netherlands. He was released on compassionate grounds in Aug. 2009 after he was diagnosed with terminal cancer and returned to Libya.
Yet doubts have persisted about Al Megrahi's conviction, and it's never been established who ordered the attack.
Robert Black QC, Professor Emeritus of Scots Law at the University of Edinburgh, tells ABC News that "I do not believe Megrahi was guilty. Certainly, on the evidence led at his trial he should never have been convicted".
Jim Swire, who lost his daughter Flora in the bombing, tells ABC News that he continues to believe that Megrahi had no involvement.
Was Al Megrahi Involved?
For the doubters, questions remain about the reliability of prosecution witnesses, the handling of forensic evidence, and even whether Libya was behind the attack.
At first Libya was not seen as a prime suspect, according to Britain's domestic intelligence service, MI5. Initial suspicions fell on a pro-Palestinian group based in Syria.
That changed after a breakthrough in the case that eventually led investigators to Megrahi.
A painstaking forensic examination of the debris from the Boeing 747, scattered across 800 square miles of Britain, found traces of explosive in a luggage container, and identified a suitcase which had contained the bomb.
Investigators then found fragments of clothing classed as "category one blast-damaged", meaning they were inside the suitcase containing the bomb.
The clothes were traced to a store in Malta, where the storekeeper recalled selling the clothing to a man resembling Al Megrahi.
It was found that the suitcase had been loaded onto PA103 from a connecting flight from Frankfurt, where records suggested that one item of luggage had been loaded on to the aircraft from a flight out of Malta.
Evidence was later heard in court that Megrahi worked for Libya's intelligence service, and until January 2007 was head of its airline security section.
It was shown in court that Megrahi travelled to Malta in Dec. 1988 using what's known as a "coded" passport, meaning a passport in a false name but issued by the Libyan passport authority.
Secret evidence, seen only by the trial judges, further implicated Libyan intelligence and a Libyan Airlines official in the operation, according to a former MI5 officer.
Among other findings made public was a tiny fragment of electronic printed circuit board identified by MI5's main explosives and weapons expert as coming from a long-delay Swiss made digital timer.
The manufacturers said they had supplied the same type of timing mechanism to Libya.
However a review of the case by the Scottish Criminal Cases Review Commission found in 2007 that Al Megrahi may have been wrongfully convicted.