The true extent of Peter Tobin's killing spree may never be known. As police trawl the records of missing women in an attempt to discover who else he targeted, Detective Superintendent David Swindle of Strathclyde police admitted that the serial killer could take his secrets to the grave.

The 63-year-old was convicted of his third murder last Wednesday – that of 18-year-old Dinah McNicol, whom he kidnapped and drugged. McNicol had disappeared in August 1991 after hitchhiking home from a music festival in Hampshire.

Tobin was already serving sentences for the murders of two women before being convicted of McNicol's murder. Her body was discovered two years ago, bound and gagged, wrapped in 16 heavy-duty refuse bags and buried under concrete in the garden of Tobin's former home in Margate, Kent. A few metres away, cut in two and also wrapped in bin bags, was the corpse of Vicky Hamilton, 15, whom Tobin had abducted, raped and murdered in Bathgate, West Lothian, in February 1991.

Tobin has also been convicted of the murder of Angelika Kluk, 23, a Polish student he killed in September 2006, dumping her body under the floor of a Glasgow church.

It was his arrest in connection with Kluk's murder in 2006 and the discovery that he had moved around the country using alibis that prompted all forces to re-examine cold cases.