Men who are expert at luring away children

When children learn about ''stranger danger'', they are told not to accept sweets from men that they do not know. In reality, paedophiles are likely to be known by their victims and often do not even need a bribe to lure them away.

Some of the nation's most dangerous offenders are "seductive abductors" who are experts in snatching children. These include Robert Black, who is currently serving 10 life sentences for killing three young girls, and Roy Whiting, the man who abducted and murdered Sarah Payne.

We cannot know if Holly Wells and Jessica Chapman too were abducted by people they trusted and thought they knew, but if they did, they would have fallen victim to a long-established technique.

The seductive abductor creates a false emergency in the minds of the victims, for example telling them that a relative has been rushed to hospital, or by posing as an official figure, such as a police officer. They may even focus on an item of clothing worn by the child as a way of forming a connection with them.

Ray Wyre, a leading expert on sex abusers, has spent more than 30 years dealing with hundreds of Britain's most notorious sex offenders and has spent hours listening to their depraved fantasies.

He has gained unique insights into the perverted minds of men such as Robert Black. "They create the illusion that the car is a safe place," he says. "Paedophiles are experts at this. I dealt with one offender who managed to get at least 100 boys into his car over a number of years before anyone reported him."

Black's method was to pull his car up on to the side of the road and pretend to be working under the bonnet. Any child who came along would have to squeeze past him and the vehicle. And in that moment he would carry out his abduction.

"He was so aware of his environment," says Mr Wyre. "In his head, he had an abduction 'box', a mental image of the location for an abduction."

When a young child is abducted, Mr Wyre lies awake wondering if one of his former clients is involved. "The problem with my work is that every offender you've ever worked with starts going through your mind, every scenario, every different way that offenders operate," he says.

He first became involved with sex offenders through his work with the Probation Service in the 1970s.

He helped set up the first residential programme for sex offenders, the Clinic for Sexual Counselling and the Gracewell Institute and Clinic in Birmingham.

"I was working with people who had been abusing for 20 to 30 years without being caught – how did they do that? How did they control the environment they abused in?" said Mr Wyre.

The answers to those questions have proved vital in helping police forces across the world investigate sex crimes, especially those against young children.

This has been achieved at no small personal cost to Mr Wyre. His clinic has been the target of arson attacks and he has been called a ''voodoo worker''. One of his former clients was also shot in the head and killed by vigilantes.

However, he is stoical – the price of not dealing directly with sex offenders is always paid by the victims.

"When I began, dealing with sex offenders was not the done thing. It was the non-intervention of our countries that lead to the victims paying the price, and yet politicians would say to me, 'There are no votes in this'. I had dinner one night with David Mellor (the former Tory Home Office minister) who said just that."

So how should society deal with those rare but dangerous individuals whose obsession in life is to sexually abuse and murder children? In response, he recounts a conversation with Robert Black.

"I asked him how could parents stop him doing what he was doing. He said you would have to chain the child to the mother and then said even then he would probably take the mother as well."

In Mr Wyre's view, Black could have been prevented from killing three girls if he had been dealt with at an early age.

"Punishment on its own will never be enough. At any one time there are more offenders in the community than in prison and there have to be programmes to deal with them," he says.

"There is such a variety of sex crimes, but the community will never see the difference between a man who exposes himself and a man who kills."

A wide raft of measures is needed, he says, from the setting up of special sex crime investigation units to permanent identification (PIN) numbers for people who work with children. There also needs to be a change in how children are educated about 'stranger danger'.

"Most children think you don't take sweets from strangers because the sweets are poisoned. If you say it's because that stranger wants to touch your willy, they burst out laughing," says Mr Wyre.

"Children need to be told what a stranger is. You are no longer a stranger after a second. I wish people would stop saying abduction is done by people they know – it's not, it's done by people they thought they knew."

How the police kept the story in the public eye

By James Morrison, Arts and media correspondent

The one unqualified triumph of the police operation surrounding the disappearance of Holly Wells and Jessica Chapman has been its handling of the media.

From the moment a fortnight ago when they realised they were facing a major incident, Cambridgeshire police have issued a stream of updates and headline-grabbing witness appeals.

Within hours of the girls' disappearance, police were keeping the media alert and on side. Details of the girls' family backgrounds and last recorded movements were let slip, generating an appetite for more concrete leads, and an expectancy that they might be in danger.

In the absence of other major news stories, an almost daily drip-feed of photographs and CCTV footage guaranteed saturation TV and radio coverage and an all-but-uninterrupted run on the front pages of most national newspapers.

No sooner had it emerged that the best friends were avid Manchester United fans than David Beckham was making a televised appeal for them to return home from what, at the time, was thought to be a misadventure. On Friday night, as the full gravity of the situation became apparent, he made a more sombre appeal alongside his manager, Sir Alex Ferguson, and team-mates.

An extraordinary moment came last Wednesday, when Detective Superintendent David Beck, who is leading the inquiry, issued a 30-second TV appeal urging the supposed kidnapper to call him on his mobile phone.

If anything, the police media machine has at times been too efficient for its own good. Last week, it came close to exposing deficiencies in the criminal investigation, when it drew attention to the fact that a taxi driver's account of two children struggling in the back of a green car had only latterly been followed up in detail.

But even in the face of mounting criticism of the detectives running the case, press officers managed to allay the situation. No sooner had questions about police competence been raised than the children's parents emerged to publicly praise police conduct.