Promise of sex failed to win confession: The Rachel Nickell undercover operation, expected to take a few weeks, lasted seven months. Stephen Ward traces its steps

THE YOUNG policewoman had been undercover for seven months, posing as 'Lizzie James' the girlfriend of a lonely man suspected by police of stabbing Rachel Nickell 49 times in front of her infant son. She had saved every letter, secretly tape-recorded their every word to each other.

Police and the psychologist supervising the operation had expected the suspect to confess to her within a few weeks, but he had not. At last they played her final card. They knew he had never had sex, never had a girlfriend. She told him she would sleep with him and be with him for ever, but only if he was the Wimbledon Common murderer. 'I'm sorry,' he told her, 'but I'm not.'

The police were left clutching a tenuous case based on psychological profiling, which was thrown out yesterday. Its details emerged only during the prolonged legal arguments at the committal hearing earlier this year, and in the High Court over the past fortnight, but could not be reported until today.

So how had they been sucked into an operation which the judge ruled could not have been used in court even if it had worked?

Rachel Nickell, 23, was killed on 15 July 1992, walking with her son Alex, two, and her labrador on Wimbledon Common. She had left her car just before 10am in the car park. An hour later her body was found 200 yards away, under a tree, Alex clinging to her arm saying: 'Get up mummy.'

The victim's youth, the presence of her child at the scene, the fact it had been in broad daylight and the widespread fear it engendered put extraordinary pressure on the police to catch the killer.

The investigation, led by Detective Superintendent John Bassett, was the biggest murder hunt London had seen in recent years. Thirty-two men were arrested and released and 548 suspects were ruled out. As the year ended, police had a suspect: Colin Stagg, now 31. He lived alone across the A3 from Wimbledon, his dog Brandy his only companion as he spent his time doing odd jobs, paper rounds and walking on the common. His brother Anthony had been jailed for five years in 1986 for raping a 19-year-old girl walking her dog on Putney Heath, south London.

Police had questioned Mr Stagg for three days in September. He gave a full account of his movements, but denied all involvement in the killing. At his home, on the Alton Estate, in Roehampton, police found a pentangle and two circles of stones, and a bookshelf with titles such as Cult and Occult, Sixth Sense and Earth Mysteries. His bedroom was painted black. They seized a black sheath knife, a black leather-studded belt and a pair of black gloves. There was nothing to link him to the murder and the police released him.

In December police got what they thought was a break; a woman who had exchanged letters with Mr Stagg more than a year earlier, after he had replied to a lonely hearts advertisement, recognised his photograph in the paper as a suspect in the Nickell case. The third letter had been so sexually explicit she had immediately ended the correspondence.

Such a savage murder justified extraordinary tactics, the police decided, and were backed by the Crown Prosecution Service. The police showed the letters to a forensic psychologist, Paul Britton, who had already profiled the sort of man he thought had killed Miss Nickell. The murderer would favour a young woman to use as an instrument for his sexual gratification in an act devoid of affection. He would want to exercise sexual control. The sex was likely to involve anal and or conventional intercourse. He would be excited by the victim's fear, submissiveness and acquiescence.

None of these were unusual except the use of the knife, which according to Britton was extremely rare, a characteristic of only one man in hundreds of thousands. With the police, he devised a plan for a policewoman to pose as a like-minded sexual deviant, to gain Mr Stagg's confidence. The Crown said in court that the intention was not to try to gain a confession but to try to draw out Mr Stagg's 'extreme sexual personality'.

Mr Britton predicted before they began that the ploy would take between 2 and 16 weeks to work. During this time, the fantasies would develop and merge into the facts, and an admission that he was the murderer. If the suspect ever diverged from the expected profile, he would have cleared himself, and the arrangement would be called off.

Until a decade ago any undercover operation was seen in law as subterfuge, denying a suspect his right not to incriminate himself unknowingly. The 1984 Police and Criminal Evidence Act tacitly admitted that there were times when covert operations were the only way to secure evidence. But it strictly limited the extent of these. The evidence must not be an interrogation, must not be prolonged unduly, and it was for each trial judge to decide the merits of the case. The most spectacular example ruled admissible came to court in 1992, and was upheld on appeal. Police had opened a jewellers shop in north London, 'fencing' stolen goods. They filmed all the customers, traced some of the goods back to burglaries, and a number of prosecutions resulted.

But earlier this year in Leeds a case came to court where an undercover policewoman had befriended a 38-year-old grocer suspected of murdering his wife. On a tape recording presented to the court, he told her he was the murderer, but the judge ruled the 'confession' inadmissible. The case was dropped.

As the Stagg case progressed into deeper and murkier legal waters, the police checked regularly with the Crown Prosecution Service to see whether what they were doing was likely to be admissible. They were repeatedly told to carry on.

A suitable policewoman was chosen: blonde, 30, and given the pseudonym Lizzie James. She wrote to Mr Stagg saying she was a friend of the shocked previous correspondent, but different in outlook. She described herself as blonde, attractive and 30. She had read the 'obscene' letter and found it interesting.

He was suspicious, believing the police had been trying to 'fit him up' for the Nickell murder. Nevertheless, he wrote back the same day. In the letter, he said he did a lot of nude sunbathing on the Common in a secluded spot.

Her letter back to him was already making sexual innuendo. 'As I've not had a relationship with a man for a long time, I sometimes long for company, the company that only a man can give.' He wrote to her that he had never had a sex life. He included in his letter a fantasy that includes having sex in the garden.

She sent him a Valentine card, and on 16 February he thanked her and said he hoped the fantasy had not put her off. He said he had an open mind about sex. He sent a further fantasy letter in which the two had sexual intercourse and she replied with more encouragement.

He sent further fantasies, then worried that he had put her off. 'Please please please don't worry about anything you've written to me. I understand exactly how you feel only I'm a bit more wary than you. But I'm feeling more confident every time you write.'

Until 28 April they corresponded, and he revealed nothing to match the profile of the Nickell killer, so she began speaking to him on the telephone as well. He still said nothing of any use. In May they met in Hyde Park for a picnic. Gradually she encouraged him to think that she liked unusual sexual practices and to be dominated. She said she had taken part in a satanic murder when she was a teenager, involving cutting the throat of a woman and baby, and that it had been followed by the best sex she had ever had. Far from following suit, he said the murder of the baby was particularly repugnant to him, and he thought all human and animal life was sacred.

She said she could only have a relationship with somebody who had had the same experience as her. Instead of confessing to the Nickell murder, he pretended he had taken part in a fictitious murder in the New Forest. The nearest he came to a confession was to say he had been on the common when she died, after telling police he had not. He said he would take her to the murder scene, but took her to the wrong place. He told her the victim had been raped when she had not.

Finally in July she pushed the strategy to the limit, threatening to break off the relationship unless he was the man who had killed Rachel Nickell.

His counsel argued that a confession obtained under such duress would have been one of the most suspect ever offered in a court. But even to keep the woman he had become obsessed with, he declined to confess as much. At the end of July the 'relationship' ended. In August he was arrested and charged with murder.

(Photographs omitted)