The night of Sunday, February 10, 1991, was atrocious, with snow lying thick on the ground. At a bus stop in the centre of Livingston, a sprawling town off the M8 between Glasgow and Edinburgh, a nervous 15-year-old girl hugged her big sister and got ready to say goodbye. Vicky Hamilton had been spending the weekend with Sharon, and now she was going home to Falkirk. She had never been away on her own before and was anxious about having to change buses on the way back.

She had gone to a call box and phoned her father, because he had a car — but he was not in. Her uncle John was not in either. When the bus pulled up cautiously on the fresh snow at about 5pm, the girls waved goodbye. And Vicky Hamilton, in her black bomber jacket and jeans, wearing the jewellery she had borrowed from her mother and carrying the dreams and dilemmas of every teenager, set out on a journey that was to last for more than 17 years.

Sharon, scurrying back to the warmth of her flat, would never see her sister again. In Bathgate, Vicky got off the bus. She was, in the words of Keith Anderson, the detective who led the inquiry after her disappearance, “a pretty lonely, insecure wee girl on that journey”. In the chip shop she asked for directions to the next bus. She asked at least seven other people, four of them men.

Hanging around in the centre of Bathgate that evening was a seedy former council roadworker-turned-car mechanic, whose name was Peter Tobin.

Scotland’s — possibly Britain’s — biggest missing person inquiry was news for almost two decades; many grew up with the mystery. The open, smiling face, the sassy black bob — Vicky was someone the Scots felt they knew well.

It is only now, after a police investigation costing millions and a trial lasting four weeks, that the story can be told. And the horror of it is undiminished by time.

Less than a year before Vicky disappeared, Tobin’s wife had fled his violence, taking their toddler son. Tobin took an overdose a month later, and declared himself bankrupt. It seems that a predatory Tobin, still resentful towards women, spotted Vicky. Did he act on impulse or did he go looking? We can never know. Either way, the weather was on his side: cold, dark, snowy.

Mr Anderson, now retired from Lothian and Borders police, said: “There are huge gaps about what happened that night. My belief is that she was abducted from Bathgate town centre. She may have been grabbed.

“However, what you have to consider is the weather conditions and the fact Vicky Hamilton was very insecure in her behaviour and she was already asking strangers for reassurance and advice. It may have been that she spoke to him and went willingly into his car.”

Very quickly, detectives believe, Tobin took her to 11 Robertson Avenue, a mile from the town centre. Perhaps he made an excuse, telling her that he would give her a lift home. Once inside, he probably gave her a hot drink containing his prescription antidepressant amitriptyline, which he knew acted as a sedative in large doses.

The post-mortem examination showed that the drug was present in Vicky’s liver, suggesting that she was kept alive for a few hours. Tobin pinned her on the floor, compressed her neck, hit her and sodomised her. She fought, and she was a sturdy girl, but he overpowered her.

“Four to six hours — that was the extent of Vicky’s life after she came into contact with Tobin,” Mr Anderson said. “When we recovered the remains, she was partially clothed [which] means, I think, she met her death very quickly.”

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