A retired chief constable once said "you couldn't make up" the story of how the Scottish and US investigation discovered key Lockerbie bombing witness Tony Gauci.

Charred fragments of clothes found in the countryside around Lockerbie were traced to a manufacturer on Malta, then Gauci's small nondescript shop in Selima.

The shopkeeper - who died last Saturday - told astonished Scottish officers that in the weeks before Pan Am 103 was brought down, he sold a seemingly random selection of clothes to a Libyan.

The inquiry established that clothes from Gauci's shop had been inside the suitcase with the bomb.

However, his identification of Megrahi as that Libyan took many months to establish and was never 100% absolute.

Judges at the trial decided Mr Gauci was a credible and reliable witness, but at the time and ever since his testimony has been bitterly contested by Megrahi's supporters, including some relatives of the British victims of the bombing.

Gauci's evidence was only part of the case against Megrahi but without it, it is highly debatable whether he could have been convicted.

Megrahi's supporters believe that the Crown case would have fallen apart. Efforts to win Megrahi a posthumous third appeal have not succeeded so far.

Unsurprisingly, prosecutors have always stood by their case and they are continuing their efforts to bring other suspects to trial.

Megrahi was convicted on the basis that he was involved in the bombing with other members of the Libyan intelligence service and in 2015, Scotland and the US asked Libya for permission to interview two men in connection with the atrocity.

The then Lord Advocate Frank Mulholland and US Attorney General Loretta Lynch said there was a proper basis in law to treat the two men as suspects.

They are Colonel Gaddafi's former intelligence chief Abdullah Senussi and Abouajela Masud, a convicted bombmaker. Both are in prison in Libya. Senussi has been sentenced to death.

Before he left office in May 2016, Mr Mulholland told STV News a second trial was "a realistic possibility," a claim treated with scepticism by campaigners like Dr Jim Swire, whose daughter Flora died on the plane.

If it did happen, a second trial will not be able to hear first hand from the Maltese shopkeeper whose evidence helped convicted Mergahi of the worst mass murder in British legal history.