Detectives are preparing to interview Robert Napper again in Broadmoor amid speculation that Rachel Nickell’s killer might have taken other lives during his series of violent crimes in the early 1990s.

Napper, 42, admitted the manslaughter of Ms Nickell, 23, who was stabbed 49 times as she walked on Wimbledon Common with her son in 1992, at the Old Bailey this week.

He had previously been convicted of killing Samantha Bissett, 27, and her daughter Jazmine, 4, as well as one rape and two attempted rapes. He has also been linked to a series of sex attacks in southeast London.

Napper has refused to talk to police about any of his crimes unless officers provide incontrovertible evidence of his guilt. But officers hope that a court-room apology he offered to the Nickell family and medical reports suggesting that he is increasingly aware of his mental illness might prompt him to volunteer further disclosures.

A senior Scotland Yard source said: “We have studied Napper in exhaustive detail and found no firm evidence to convict him of outstanding offences. But there may be other crimes in the system for which he is reponsible – there are gaps in his period of offending which have puzzled us.

“The worst-case scenario is that he has raped someone who has never come forward or that he has killed someone and buried the body, although that is not his methodology. We’re working very closely with Broadmoor on the best way to approach the interview.” A forthcoming book, Killer in the Shadows, co-written by Laurence Alison, a leading forensic psychologist, examines whether Napper might be linked to the murders of Claire Tiltman, 16, Penny Bell, 43, and Jean Bradley, 47, between 1991 and 1993.

Miss Tiltman died in January 1993 in Greenhithe, Kent, as she took a short cut down an alley on her way to visit friends. She was stabbed more than 40 times.The attack came between the deaths of Ms Nickell and Ms Bissett and was followed two months later by the death of Miss Bradley, a businesswoman who was stabbed more than 30 times as she got into her car near Acton Town Underground station in West London.

Her death bore strong similarities to the murder, in June 1991, of Penny Bell who suffered more than 50 knife wounds when she was attacked in her car in Greenford, West London.

The murders have a number of similarities to Napper’s killings of Ms Nickell and Ms Bissett and her daughter – notably the frenetic violence. But there are also differences, including the apparent absence of sexual assault.

Miss Tiltman was attacked in a narrow pathway, as were some of Napper’s rape victims, but there have been two other strong suspects for her killing, including a convicted sex attacker who lived locally and a drifter who committed suicide the following year after killing a young woman in Devon.

Miss Bell and Miss Bradley were blondes, like Ms Nickell and Ms Bissett, and died in West London, where it is now known that Napper had planned and committed attacks. But both women were older than Napper’s other targets and were attacked in their cars, while he tended to strike in parks and on commons.

In his book, Professor Alison writes that no one should leap to conclusions about Napper’s role in the killings. “When we look into why killers have or have not been locked up for their dreadful crimes, we also need to remember a centrally important fact. The very essence of our justice system can be explained thus: it’s not necessarily about what is true, it’s about what we can prove. The sad fact is that these cases remain unsolved – there isn’t the proof. That is separate from what is true, of course. It may not be true that Napper killed others, it may be.”

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