He has already been convicted of three murders and the search to identify more victims is on after Peter Tobin reportedly told a prison psychiatrist that he killed 48 women.

He has already been convicted of three murders and the search to identify more victims is on after Peter Tobin reportedly told a prison psychiatrist that he killed 48 women.

Police across the country are not taking any chances.

As part of Operation Anagram, a nationwide investigation to

determine whether Tobin can be linked to any of Britain's unsolved murders, they are looking into the disappearances of six women in Norfolk, where the killer regularly holidayed.

No one has ever been charged with the deaths or disappearances of Pamela Exall, 21, Yvette Watson, 17, Natalie Pearman, 16, Johanna Young, 14, April Fabb, 13, or Susan Long, 18, and although officers say there is no evidence to link them with the 63-year-old from Renfrewshire, Scotland, Professor David Wilson, a leading criminologist from Birmingham City University, says he is almost certain that Tobin, who is now serving three life sentences in prison, struck in Norfolk at least once.

'Several of the Norfolk cases were soon found to contain his hallmarks,' he said.

'He did not start killing in 1991, when Dinah McNicol was killed. Serial murder is a young man's business, and people tend to start to murder in their late teens or early 20s.'

Prof Wilson has conducted extensive research into the topic, including interviewing detectives who worked on the original cases and speaking to the killer's ex-wives to better understand his behaviour, and said the exercise revealed Tobin to be 'a sexual sadist'.

The unsolved 1969 case of April Fabb, a 13-year-old girl who disappeared while on a bike ride from Metton, near Cromer, to Roughton, is one of those in Norfolk that the professor thinks could be attributable to Tobin.

'Tobin was an opportunist murderer,' he said. 'He hadn't planned to pick up Dinah McNicol in 1991 - he had just spotted her waiting at the side of the road and had taken advantage of the fact.

'A young girl running a birthday errand on her bike might have proved equally irresistible to him.

'Furthermore, the abduction took place on Easter Monday, which fitted with Tobin's habit of taking bank holiday breaks in Norfolk.'

Former head of Norfolk CID Maurice Morson, who took on the Fabb case in 1983, published a book in 2007 called April Fabb: The Lost Years, in the hope it would lead to fresh information as the investigation reached its 40th anniversary, but police have never charged anyone with the killing.

Another case Prof Wilson thinks Tobin may be responsible for is that of Susan Long, who was murdered in 1970 in Aylsham after getting the bus home from an evening's dancing at the Gala Ballroom in Norwich.

'The killer left traces of his blood on Susan's clothes,' he said. 'But despite this and an extensive investigation at the time, he was never caught.

'Tests were conducted on the blood found on Susan's clothing, and a forensics team was able to create a DNA profile of the murderer.

'The local police then revisited a number of people who had been questioned during the original investigation.

'But perhaps the killer had not lived locally. Perhaps he had merely been on holiday in the area - as Peter Tobin was on numerous occasions.'

He is also convinced that Tobin is the unknown serial killer dubbed 'Bible John' by police after he killed three women in Scotland in 1968 and 1969.

Bible John killed Patricia Docker, 25, Helen Puttock, 29 and Jemima McDonald, 32 - all of whom he met at the Barrowland Ballroom - after assaulting them and strangling them with their own tights. Despite one of the biggest manhunts in Scottish police history, the crimes remain unsolved for more than 40 years.

Prof Wilson said that by crossing police force boundaries, Tobin made it hard to link up a series of murders. He also hid the bodies of his victims, which allowed him to put distance between him and his crimes.

The killer has been convicted of the murders of Angelika Kluk, Vicky Hamilton and Dinah McNicol, but his whereabouts in Norfolk at the time of the other women's disappearances are still unknown.

Prof Wilson added: 'Norfolk is an important hot spot in relation to Operation Anagram.

'I'm confident that in due course, the police will find there's evidence to link Tobin to the murders that took place there.'

A spokesman for Norfolk police said: 'All of our investigations relating to unsolved murders and missing people remain live inquiries and we always welcome any new information from members of the public in relation to any of these matters. The potential link with Peter Tobin has been investigated and there is no evidence at this time to suggest a link between him and offences in Norfolk.'

Anyone with information about any of these cases should contact Norfolk Police on 0845 456 4567 or call Crimestoppers anonymously on 0800 555111.

David Wilson's new book, co-written by Paul Harrison - The Lost British Serial Killer: Closing the Case on Peter Tobin and Bible John - is published by Sphere, priced at �6.99.