An anonymous letter claiming to identify the gunman who murdered an Indian waiter in a close-knit island community was passed to police 12 years after the shooting, a court was told yesterday.

The note, which was handed to police in Kirkwall, Orkney, in September 2006, said that the killer of Shamsuddin Mahmood, a 26-year-old Bangladeshi, was around “15+” years old and had been spotted by the witness coming out of a cubicle in a public lavatory. It signed off: “Worried sick witness.”

“This is a true letter, I promise that I saw the person who killed the Indian waiter,” the note began. “I saw his face in full and the handgun. It was in toilets at Kiln Corner. I have lived long enough with the guilt at not coming forward. The person was about 15+ years approx - white and had a balaclava on [his] head but still not turned down. The colour was either dark blue or black and dark clothing.

“He came out of the cubicle but went back in quick when he saw me. I looked over and saw his face in full. “The handgun was natural polished metal or silver and a big Beretta. This may seem stupid, but the way he held the handgun looked like he had held a firearm before. I just don't ken what to do. Worried sick witness.”

The existence of the letter was disclosed as the trial of Michael Ross, 29, of Inverness, entered its second week at the High Court in Glasgow. Mr Ross, a sergeant with the Black Watch, whose father was a police constable who attended the murder scene, denies killing Mr Mahmood at the Mumutaz Indian Tandoori restaurant in Bridge Street, Kirkwall, on June 2, 1994. Susan Sinclair, 27, a police civilian worker, told the court that the note was handed in to the police station in Kirkwall on September 2, 2006, by a man called William Grant. She said that she recognised Mr Grant because her friend worked with his wife at a local hotel.

Ms Sinclair said: “I opened the letter and read it and passed it on to Detective Inspector Petrie.” Asked whether she spoke to Mr Grant, when he delivered the letter, she replied: “I don't really recall what was said. He handed me a letter and then he just turned and walked out.”

She agreed when asked by Brian McConnachie QC, for the prosecution: “You don't know if he was the author or the postman.” Mr Ross, who was 15 at the time, is accused of entering the restaurant with his face masked and shooting 26-year-old Mr Mahmood - known as Shamol - in the head.

He is also accused of attempting to defeat the ends of justice by changing his clothing and disposing of the weapon. He is further charged with, while acting with others whose identities are unknown, committing a breach of the peace outside the Indian restaurant by shouting, swearing, uttering threats of violence and racist abuse. The offence was allegedly committed between May 3 and May 24, 1994. He denies all charges and has lodged a special defence of alibi claiming that he was nowhere near the Indian restaurant or Kirkwall town centre, but was cycling in another part of Orkney.

Earlier yesterday, a pensioner described how only minutes after the murder he had heard banging and rustling coming from a cubicle in the public lavatories at Kiln Corner in Kirkwall. John Rendall, 74, a retired telecoms engineer, told the court: “There was a lot of rustling of paper or clothes, banging as though moving about, touching the sides. Not the sort of noise you would generally expect to be coming from a toilet.” He said that by the time he came out of another cubicle the person had already left the lavatories and was walking away down the street.

After hearing on the radio that there had been a murder, he said that he told his wife: “That's probably the [person] that I heard in the toilet.”

Another witness, Sarah Richards, 32, told the court that she caught a glimpse of a man, whom she thought was aged about 30, running from Petmania Lane, Kirkwall. However, she was unable to identify him at an identity parade. The trial, before Lord Hardie, continues.