It was Vicky Hamilton's purse which provided detectives with their badly needed "eureka moment" in linking the teenager to Peter Tobin.

It was Vicky Hamilton's purse which provided detectives with their badly needed "eureka moment" in linking the teenager to Peter Tobin.

Her disappearance in 1991 had sparked the biggest missing persons inquiry in Scotland. More than 7600 names were linked to the investigation and almost 4000 statements taken by officers. Five hundred relevant documents had been filed and 300 items of evidence stored.

But the black leather purse, which sat ignored in a police storeroom in Livingston for more than 15 years, offered a breakthrough in the case after it became one of 21 new lines generated by the cold case review launched in 2006. It was the only possession of Vicky's that police knew had been with her at the time she went missing.

Developments in DNA technology meant the purse could be sent to the Lothian and Borders Police forensic laboratory at Howdenhall in Edinburgh on December 19 that year. Four months later, a DNA profile was found. It was not the genetic match of either Tobin or Vicky Hamilton and a search of the DNA database gave a negative result.

On June 1, 2007, police learned that it almost certainly belonged to Daniel Wilson, the son of Tobin, with the odds against being one in a billion.

The boy, just three at the time of Vicky's death, had stayed with his father in Bathgate the week after the teenager vanished. He had apparently been allowed to put the purse in his mouth, since the sample was DNA-rich and likely to have come from saliva.

Retired senior investigating officer Keith Anderson, of Lothian and Borders Police, said: "Without a doubt, it was the eureka moment. That was the breakthrough, the thing that really gave us the impetus to move ahead."

Around the same time, Detective Superintendent David Swindle of Strathclyde Police, who was researching Tobin's previous addresses ahead of the Angelika Kluk case, noted that he had lived in Bathgate. This information was passed to DCI Anderson.

The match on the purse came just two days before police moved into Tobin's home in Bathgate for a deep forensic search of the property. Investigators entered the loft to clear what they thought were stacks of household rubbish for tests, but came away with something far more significant.

A hunting style knife was found wedged between the dividing wall of the property and the first floor joist in the attic. On June 15, 2007, a tiny patch of skin pulled from the blade by scientists matched the profile from Vicky Hamilton's Guthrie Test - drops of blood taken from a newborn to detect childhood diseases - which had been found in the archive of the Royal Hospital for Sick Children at Yorkhill, Glasgow.

Mr Anderson said: "June 15 was a day of much whooping and hollering. After that, the investigation took off like a runaway train."

But the lapse of time between Vicky's disappearance both helped and hindered the reinvestigation. While officers were able to take advantage of the DNA technology, strong documentary evidence and personal recollections were harder to come by.

"People's recollections did fade with the passage of time. Recovery of documentary evidence was nigh on impossible. Bank records, DSS records, court records, unfortunately these get destroyed after seven years. It was very difficult to obtain anything," said Mr Anderson.

However, the cold case review had turned the latent missing person's inquiry into a murder investigation, a step which meant officers could "move off the fence" and into new ground, according to Bert Swanson, the former DCI who managed the review and decided that Vicky's disappearance merited reinvestigation.

He did the same with the World's End murder of 17-year-old Christine Eadie and Helen Scott in 1977, although the case against Angus Sinclair dramatically collapsed last year due to a lack of evidence.

Speaking of the Vicky Hamilton case, Mr Swanson said: "The investigation at the time never treated Bathgate as a crime scene. The parameters put in place were very, very restrictive. In a missing persons inquiry, you speak to a lot of people and show them a photo.

"It was my opinion that Vicky was not a runaway. After spending six months going through a vast amount of evidence, there was no information to support that. If the truth be known, a lot of people, including those who were working on the case, thought she had run away. That was the strong opinion of the senior investigating officer at the time."

Mr Swanson said there was no record of officers carrying out door-to-door inquiries in Robertson Avenue, where Tobin lived and Vicky died.

He added that the street, as well as others in Bathgate, merited another look, given the proximity to Bathgate town centre, where the teenager was last seen alive. "It is my belief that Peter Tobin did feature as an unidentified person in that respect. He was never spoken to," Mr Swanson said.

But Chief Superintendent Malcolm Graham, now head of Lothian and Borders CID, defended the initial police action, saying that at the time of the missing persons inquiry, Tobin had no convictions and there was no information to lead police to him.

"In 1991, people did not have mobile phones and CCTV was not prevalent in terms of identifying people," he added.

Asked if there had been a missed chance in identifying Tobin while he still lived in Bathgate, Mr Graham said: "Absolutely not. There was no information to lead us to Mr Tobin's door."