May 20, 2012 -- Libyan intelligence officer Abdel-baset Ali Mohamed Al Megrahi, the only man convicted in the 1988 Lockerbie bombing died at home in Libya today, according to his brother. He was 60.
Yet doubts have persisted about Al Megrahi's conviction, and it's never been established who ordered the Dec. 21, 1988, attack, in which a bomb exploded onboard Pan Am flight 103 from London to New York.
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.
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 August 2009, after he was diagnosed with terminal cancer, and returned to Libya.
The release was greeted with outrage, but there are others -- including some family membvers of the victims of the terrorists' bomb -- who question whether Megrahi should ever have been in jail at all.
"I do not believe Megrahi was guilty," Robert Black QC, professor emeritus of Scots Law at the University of Edinburgh, told ABC News. "Certainly, on the evidence led at his trial he should never have been convicted."
Jim Swire, who lost his daughter Flora in the bombing, also said 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, which was scattered across 800 square miles of Britain, found traces of explosive in a luggage container, and identified a suitcase that had contained the bomb.
Investigators then found fragments of clothing classed as "category one blast-damaged," meaning they were inside the suitcase that held 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 December 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.
Among the grounds for doubt was new evidence indicating al Megrahi may not have been in Malta when the clothing was purchased. It was also revealed that four days before the ID parade at which the Maltese store worker picked out al Megrahi, he had seen a photograph of the Libyan in a magazine article linking him to the bombing.
Al Megrahi insisted on his innocence right up to his death. Some of his supporters claim he may have been a convenient scapegoat.
Libya's leader Muammar Gaddafi was a longstanding supporter of terrorism around the world, but by the late 1990s wanted to end his country's pariah status.
After lengthy negotiations, Libya accepted responsibility for "the actions of its officials" over Lockerbie, agreed to pay more than $2 billion in compensation to victims' relatives, and surrendered Megrahi to the Scottish authorities for trial.
Libya gave up its weapons of mass destruction, and shared intelligence with the United States on al Qaeda in the region. In turn, Western governments and companies were keen to exploit the country's important oil and commercial potential.
After Gaddafi's regime was overthrown, it was hoped that new information may come to light about what led to PA103's catastrophic damage at 31,000 feet above Scotland.
Secretary of State Hillary Clinton said recently that the United States will continue to pursue justice on behalf of the victims.
"The United States has kept open the case concerning the Lockerbie bombing," she said. "We want more information, and we want to have access to those who might have been somehow involved in the planning or execution of the bombing."
Last year Scotland's most senior law officer said he would continue his efforts to investigate the case.
Lord Advocate Frank Mulholland said the idea that Megrahi acted alone was "risible," and added that he would be "failing in his duty" if he did not find the people who were responsible for the bombing.