Peter Tobin's conviction for the murder of Dinah McNicol brings to an end 18 years of painful uncertainty for her family, but the unsettling fact is that McNicol's body and that of Vicky Hamilton, killed six months before her and buried just feet away from her in his Margate back garden, might never have been found had Tobin not gone on to kill again.

After an early conviction in the late 1960s for fraud, the Scot was sentenced in 1994 to 14 years in prison for a brutal attack on two 14-year-old girls in Hampshire, in which he forced them to take drugs at knifepoint before raping and buggering one and sexually assaulting the other.

He served 10 years in prison and was added to the sex offenders register, and on his release in 2004 moved to Paisley. By the following year, however, Scottish police had lost track of him, and he was posing as a homeless handyman called Pat McLaughlin when, in September 2006, he raped and murdered Angelika Kluk at the Glasgow church where she lived and worked.

Two factors about that murder were particularly disturbing to police, suggesting that Tobin may have killed before. The first was its breathtaking brutality – Tobin repeatedly bludgeoned Kluk around the head before raping her, tying her up and gagging her and stabbing her 16 times – the second the brazen disregard he showed for covering his tracks. Though he fled to London after the killing, Tobin left near the scene a pair of his jeans soaked in Kluk's blood, while his DNA was found on her body.

The fact that Kluk had not been known to Tobin before the crime, as with the 14-year-olds, and that he had buried her body were additional red flags to police.

The Kluk investigation had revealed that Tobin used numerous aliases, used 38 different mobile phone sim cards and travelled extensively throughout the UK in the year before her murder. Every police force in the country was alerted that Tobin might have killed before, prompting an urgent re-examination of unsolved cases under the codename Operation Anagram.

Strathclyde detectives discovered that Tobin had been living in Bathgate at the time of Vicky Hamilton's disappearance, and alerted Lothian and Borders police, who searched his former home in Robertson Drive in June 2007. Hidden in the rafters of the house was a dagger-like knife on which was found a tiny piece of Vicky Hamilton's skin.

Tests on Hamilton's purse, which had been found some months after she disappeared at a bus station where Tobin had planted it, showed traces of DNA found to belong to Tobin's son Daniel, who had been a toddler in 1991. It was enough to charge him with Hamilton's murder, even without a body.

At the same time, Essex police were looking again at Tobin in connection with Dinah McNicol's disappearance in August 1991, under the codename Operation Broadway. Tobin had been one of 2,000 names that arose in the initial investigation, but what they now knew about him made him an urgent suspect.

A warrant was obtained to search 50 Irvine Drive in Margate, where Tobin was living in 1991, and after forensic archaeologists using ground-penetrating radar found suspicious results in the back garden, it was dug up. When a body was found, the detectives expected it to be Dinah's; instead, it was Vicky Hamilton's. Tobin had transported the body, which, after many inexpert attempts, he had cut in two, from Scotland when he moved to Margate a month after killing Hamilton. McNicol's body was found four days later.

The two murders would normally have been tried together, but because Hamilton had been killed in Scotland and Tobin was already serving a life sentence there, the Crown Office, equivalent of the Crown Prosecution Service in Scotland, was eager to try that murder north of the border; under Scottish law, McNicol's killing could not be tried there.

The trial was held in Dundee to minimise the risk of prejudice following the Kluk case, and during the case Tobin's entry on Wikipedia was voluntarily modified following the intervention of the police, after his lawyer called the site "the most blatant contempt of court I have seen in my whole life". It has not been modified during the McNicol trial. The Crown Office had launched proceedings for contempt against the editor of the Mail on Sunday and a reporter from the paper after it published an interview with a witness in the Kluk case before the trial.