HE was a racist executioner who traumatised an island community at the age of only 15. On the evening of Thursday, June 2, 1994, Michael Ross coolly entered the Mumutaz Indian restaurant in a lane near the centre of Kirkwall. Wearing a black balaclava, and brandishing a handgun, he shot waiter Shamsuddin Mahmood in front of terrified customers.

The 9mm bullet passed through his 26-year-old victim's skull and lodged in the wall behind, while the bullet casing fell to the floor. It would be more than a decade before the balaclava, a notebook with Nazi symbols, and a deactivated machine gun would become parts of the jigsaw that finally revealed Ross as the killer.

The main piece, though, was the testimony of a fellow Orkney islander. He kept his guilty secret for 12 years, not even telling his then wife, before informing the police he had seen the killer on the night of the murder.

That evidence helped bring to justice Michael Ross, who had left the island in the interim: joining the Black Watch and serving with distinction in Iraq, where he was decorated for his bravery, and becoming a figure his two young daughters could admire and respect.

Back in June 1994, police launched a huge inquiry into the murder, backed by officers from Inverness. A cordon was thrown around Orkney's air and sea terminals. House-to-house inquiries were carried out at every address in Kirkwall, and inquiries were extended to take in the Bangladeshi communities in London and Southampton, where Shamsudden had his roots. Officers also travelled to Germany.

A range of motives were examined for the murder: racially motivated, drug related, a crime of passion, or a contract killing, but that inquiry initially turned up no fresh ideas. The only piece of real evidence officers had at that time was the bullet and its casing. PC Edmund Ross, Michael's father, was one of the first officers at the murder scene. A former Royal Green Jacket and Special Branch officer who once protected Prince Charles and Princess Diana on a visit to Sutherland, he was given the task of test-firing all 9mm weapons on the island as part of the investigation.

The murder trial heard of Mr Ross's great passion for firearms. He owned 11 guns at the time of the murder - five pistols, two revolvers, three rifles, and a shotgun - and had given Michael the deactivated machine gun as a present. He and his younger brother, Colin, now 28, were both taken out on shoots by their father.

The views of their mother, Moira Ross, are unknown. She has consistently refused to comment on the case.

PC Ross told detectives the fatal bullet was a 9mm calibre, of a type supplied to the British Army by the Kirkee arsenal in India in 1972. He was asked to check all 9mm weapons on the island, and the conclusion was that none of the weapons on the islands would be capable of firing the bullet that killed Mr Mahmood. Crucially, nowhere on the island could he find the same type of ammunition.

The only other major lead had come two days after the shooting, when a mother and daughter came forward, saying they had seen a man wearing clothes similar to that of the killer in Papdale woods two weeks before the murder.

Lynn Railston, who is now 31, described how she had seen a young man "stalking" from tree to tree as if waiting for someone. The witnesses watched him through binoculars for around three-quarters of an hour. Eventually, he took off his mask and hooded top and walked away.

But two months after the murder, with still no real breakthrough, the investigation took a significant turn. Early on August 12, 1994, Constable Ross told Detective Inspector Angus Chisholm, the deputy senior investigating officer, that he had a sealed box of the same bullets from the Kirkee arsenal, but could not remember where he got them.

Mr Chisholm, now retired, and who gave evidence at the trial, said: "It astonished me because we had been looking for these for several weeks before. That was the focus of our inquiry."

PC Ross then revealed he got the bullets from James Spence, a former Royal Marine working in Orkney as a roadsweeper. When interviewed, Mr Spence was adamant that he gave PC Ross two boxes of bullets - one unopened from the Kirkee arsenal, and another box of .22 calibre.

Three weeks later, the investigation took another twist when Lynn Railston, who had seen the man in Papdale woods, spotted him again in Kirkwall.

She recalled seeing a distinctive white design on the top underneath the blue top which he removed that day in the woods. She even did a drawing of that design for police. Police checked CCTV footage and Ms Railston identified the boy as Michael Ross. Detectives questioned Michael in the presence of his father, and said he was not the person in Papdale woods that day.

He was questioned again on December 2 and this time admitted it was him in the woods, but maintained he was elsewhere on the night of the murder. Three days later, police decided to question ammunition supplier James Spence again.

Mr Chisholm said: "Spence said that on three occasions Eddie (Ross) had approached him and spoken to him and effectively asked him to tell lies. If the police did come and ask him about bullets, he was to say there was only the one box."

The following day, police obtained a warrant to search the Ross home in St Ola. Michael Ross was detained and questioned about the murder and released without charge. But the search of the property revealed several significant finds.

Constable Bob Petrie bagged a black balaclava with holes for eyes and mouth and a deactivated 9mm sub machine gun which had been hanging on the wall of Michael Ross's bedroom.

Also found, by DC Mike Harper, was a notebook belonging to Michael Ross which had the two chevrons for corporal and the word "Ross" written on it.

The "o" of "Ross" had been drawn with a swastika inside it, and the two letters "s" like the Nazi SS. Also in the notebook was a Scottish saltire, inscribed "Death to the English", and a banner on the same page which said "Death cures all".

Three months later, his father was suspended from duty. He was later charged with perverting the course of justice, convicted on May 27, 1997, and jailed for four years at the High Court in Inverness.

During his trial Michael Ross, who had joined the Army by then, was named as a suspect. But in the absence of any new evidence, progress was impossible.

However, in 2006, a new witness, William Grant, came forward to allege he had seen Michael Ross come out of a cubicle in the public toilets in Kirkwall on the night of the murder, holding a balaclava and a pistol. At first his information was given anonymously, but Mr Grant was identified by a police clerical worker as the man who handed in a letter to Kirkwall police station. The letter claimed the writer could not live with the guilt any longer and that he knew who killed the waiter.

That development led to a cold case review into the murder and, after nearly a year, Michael Ross was arrested. By then based in Northern Ireland with the Black Watch, he appeared at the High Court in Edinburgh in January 2008 for a preliminary hearing.

He was released on bail, was forced to surrender his passport and ordered not to visit Orkney until the conclusion of the trial. Yesterday, despite the courtroom drama after the verdict, the teenage killer who shocked an island community was finally behind bars.