It is the satisfactory finale to the biggest police investigation of its kind in Scotland, which had already resulted in jail sentences for six other members of the ring. Significantly, it was the first time prosecutors had successfully proved conspiracy to gain access to children for the purpose of abuse. That allowed the sentences to reflect intent as well as actual abuse.

At the centre of the case is the internet. It was both the means of distribution of the illegal, abusive images and the means by which members of the paedophile ring were caught. The investigation was triggered by the discovery of the image of a naked boy on Strachan’s computer while it was being repaired, a chance event that highlights the difficulty in tracking down such networks unless an internet service provider or the police are alerted to suspicious material.

In this case, the men were traced through their internet conversations about sexual fantasies involving children in an operation which stretched across the world. That illustrates the unlimited potential for contact between strangers offered by the internet. It has provided paedophiles, who tend to be isolated, with a means of contacting each other. By sharing their obsession with the sexual abuse of children, they can persuade themselves that their desires are not sick but normal.

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The success of this case depended on international collaboration between police in Scotland, the FBI and the UK Child Exploitation and Online Protection Centre, but the nature of the internet makes it difficult to police. The police officer who co-ordinated the inquiry is calling for internet service providers to prevent convicted offenders using the internet. The suggestion of an internet ID for everyone to allow authorities, including the police, to monitor whether sex offenders are using the internet, however, will be seen as an unacceptable intrusion by many people who fear it will be used to spy on protest groups. In this case, it would not have stopped Rennie, who had no previous convictions and led an out-wardly respectable life.

This is the cyber equivalent of putting them on the sex offenders register – an attempt to limit their ability to re-offend by ensuring they do not work with children. Just as sex offenders can disappear off the radar, they could presumably steal an internet ID. It is not a guarantee of safety.

Yet the internet does need to be policed and internet service providers should have a responsibility to remove illegal material. The Internet Watch Foundation (IWF), the UK’s self-regulatory body for websites, has a mechanism for reporting illegal and abusive material, which can be taken off sites. There is growing concern about the infiltration of social networking sites by sex offenders.

Neither Twitter nor Facebook, two of the most popular sites, are members of IWF. That makes them attractive to paedophiles who may pose as young friends to lure children and teenagers into posting naked images or agreeing to meet. The latest example is Ashleigh Hall, a teenager killed by a 32-year-old registered sex offender she met on Facebook, where he was claiming to be 16. As in real life, the best protection against child abuse is vigilance.