A PSYCHOPATHIC sex killer has become the prime suspect in the murder of Rachel Nickell, who was stabbed to death 12 years ago as she was out walking with her two-year-old son and pet dog.

Robert Napper was sent to Broadmoor Hospital in 1995 after admitting the murder of Samantha Bisset, 28, and her four-year-old daughter.

He could now be questioned about Miss Nickell’s murder in 1992 if he is deemed fit to be seen by police. Miss Nickell, 23, a former model, was stabbed 49 times when she was attacked on Wimbledon Common in southwest London.

Detectives arrested Colin Stagg, now 41, but he was later cleared after an Old Bailey judge lambasted officers for using an undercover policewoman to encourage him into making a confession.

The Nickell case has been examined by officers from Scotland Yard’s murder review group who are working with forensic scientists using DNA tests which were not available in 1992. After news emerged that police were working on the case with new DNA techniques, Mr Stagg publicly called on police to take his DNA to match with what they had uncovered. The new tests take much longer to develop than standard DNA tests but they allow scientists to uncover old and tiny samples such as flakes of skin and sweat stains.

Mr Stagg last night told The Sun: “This is the news I have longed for. Today is the best day for a decade. I have always longed for the day Rachel’s killer could be brought to justice. Maybe that day is approaching.

“Rachel’s family deserve justice to be done — and I too deserve the chance to finally clear my name.”

Some DNA has been recovered from Miss Nickell’s clothing at the time of the attack but the work is not yet complete and a profile of her attacker is not ready. Yesterday, police sources emphasised that the questioning of Napper could be some time away. Napper was one of the suspects examined by detectives during earlier reviews of the case. At the time of his conviction, police said that they were keeping an open mind about his link with the Nickell murder.

The former suspect is under renewed investigation after research by murder squad officers aided by analysts and psychologists. Napper was sent to Broadmoor after admitting the murder and dissection of Miss Bisset, 28, a single mother who was killed in her London home.

Her daughter Jazmine was suffocated and her mother later died from a heart attack brought on by shock.

At the time of his conviction, one senior detective said that the murders had been the most violent that he had investigated.

To his colleagues Napper, who worked as a machine operator and warehouseman in Plumstead, southeast London — a short distance from Miss Bisset’s home — was said to be a bit of a loner. Robin McCarlie, his former foreman, remembers Napper as “really quiet”.

“There was nothing out of the ordinary about him,” said Mr McCarlie. “He did his work. When he was here there was no great problem. In fact, he was really boring. He kept very much to himself but I couldn’t see that he was any different from anybody else around here.” The only peculiarity that his colleagues noticed was that he was never seen parted from a small sports bag that he carried around.

It was said later that he kept an armoury of weapons in the bag. Police found that he was an obsessive peeping tom who kept notes on his targets.

Detectives found a map with his victim’s home marked on it.

Napper, who was in his late twenties, had climbed into Miss Bisset’s flat and stabbed her eight times in the neck in the hallway. Her daughter was sexually abused and then killed. He was caught after leaving fingerprints in the flat and was then linked with a series of other knifepoint attacks on women in the year before the murder on Wimbledon Common. The murder of Miss Nickell caused wide alarm across southwest London. The case has become one of a group of high-profile killings which detectives have been determined to solve.

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