Anniversaries have rolled past painfully over the past two decades and yet as the 20th year is marked on the calendar this Sunday, it is difficult to see why those who lost relatives in the Lockerbie bombing are no nearer the truth.

At the time, what was done in the first years after the tragedy to identify the men responsible was described as "remarkable". In the late 1980s and early 1990s, a unique co-operative effort between the US Justice Department in Washington and the Crown Office in Scotland assembled what they claimed to be a "cast-iron" case against two named Libyans who were said to have placed a suitcase containing the bomb on board a connecting flight from Malta.

The evidence came not from intelligence contacts, but from painstaking police work in Scotland, from old-fashioned forensic evidence, and what seemed a brilliant piece of detective work by a Scottish police officer, Detective Inspector Harry Bell.

It was Mr Bell who traced clothing found in the remains of a suspect suitcase, from Scotland to Malta. He worked from a fragment of green circuit board, smaller than a fingernail, found in the wreckage area, which scientists said was part of the bomb's timing device.

Tony Gauci, the owner of the shop where the clothing was sold, Mary's House in Sliema, Malta, was said to have remembered the Libyan purchaser well and identified him. He also volunteered the fact that the Libyan had bought an umbrella. Mr Bell called his team back at base and asked them if an umbrella had been found in the suitcase. It had. It matched up. As one of those involved put it: "At that point the hair stood up on the back of our necks."

The prosecution case against Abdelbaset Ali Mohmed al Megrahi and Amin Khalifa Fhimah was said to be relatively straightforward. Al Megrahi was head of security for Libyan Airlines; Fhimah was the airline's senior official in Malta. Both were alleged to be members of ISO, the Libyan equivalent of the KGB.

Prosecutors argued that they were instructed to arrange the bombing as an act of revenge for President Reagan's attack on Tripoli in 1986. The circumstantial evidence according to the prosecution, was "overwhelming" - al Megrahi was convicted, Mr Fhimah cleared.

However, much of the evidence, including the Crown's crucial eyewitness, has now been discredited.

What the case at Camp Zeist in the Netherlands seemingly ignored was that the initial inquiry in 1989 had in fact focused on Iran, and many still argue that the circumstantial evidence that implicated them was far more compelling.

Iran's alleged motive for carrying out the attack was assumed to be a desire for revenge for the shooting down of an Iranian civilian flight by the US warship, the Vincennes, killing all 290 passengers and crew aboard, in July 1988.

Ayatollah Khomeini, then supreme leader of Iran, vowed at the time that the skies would "rain blood" in revenge.

Those who maintain that Iran was behind the attack, say Tehran sponsored one of a number of radical Palestinian groups to carry out the attack.

The group alleged to have executed the plot was the Syrian-backed Popular Front for the Liberation of Palestine General Command (PFLPGC), headed by Ahmed Jibril.

During a raid by German police on one of Jibril's PFLP-GC cells in Dusseldorf in 1988, during which 14 people were arrested, four Toshiba cassette recorder bombs were discovered. At Megrahi's trial, the Crown argued the type of bomb used to blow up Flight 103, while encased in a similar Toshiba recorder, used a different triggering mechanism. Jibril was assassinated in Beirut in 2002.

Egyptian-born Abu Talb, who was allegedly working for Jibril, emerged as a key suspect in the aftermath of the bombing. However, when al Megrahi went on trial in 2000, Talb was called as a witness for the prosecution.

It is also alleged that Talb, who is serving a life sentence in Sweden for the bombing of a synagogue in Denmark in 1985, was paid millions of dollars by the Iranian government following the bombing.

On the sixth anniversary, Tam Dalyell, the then MP for Linlithgow, accused Lady Thatcher and President Bush senior of "suppressing the truth" to protect Anglo-American security links.

Yesterday there was another hearing at the Court of Criminal Appeal in Edinburgh - one of more than a dozen - since al Megrahi was granted leave to appeal in June 2007, following a three and a half-year investigation by the Scottish Criminal Cases Review Commission.

The latest hearing sought to address the protocol for appointing a special advocate - in lieu of the defence team - to decide the final outcome of a top secret document from an undisclosed foreign country which the Crown has seen, but which was not disclosed to the defence at the trial.

The lack of disclosure is one of the six grounds of referral from the commission which suggest the case could have been a miscarriage of justice, many of which refer to unreliability of the Crown's key witness, the Maltese shopkeeper who gave evidence about items bought in his shop.

For al Megrahi, the 56-year-old currently serving a life sentence, the delays and repeated hearings must seem interminable.

In September he was diagnosed with terminal prostate cancer and his defence team told the court he "does not have long to live". With the appeal itself not due to begin until at least the summer of 2009, it seems a cruel twist that the man indicted in 1991 could die before at least some of the remaining questions are answered.