Skipton detectives have told how they launched their biggest ever child pornography investigation to trace a Cross Hills man who posted images of abuse on the internet.

David Lee Leach was involved in an international paedophile operation, which came to light when internet watchdogs picked up postings of images of young boys on sites frequented by paedophiles.

Leach, 30, was jailed for 55 months on Friday for the creation, distribution and possession of indecent images, indecent assault and indecency with boys as young as seven from England and Thailand.

Leading the investigation, codenamed Operation Foxglove, in Skipton was Detective Sergeant Pete Martin.

He had been working on Operation Ore inquiries in Craven, part of a national crackdown on internet pornography, when these offences came to light. Because of his expertise, he was chosen to head the Operation Foxglove team.

Images of young boys - known as MMR pictures - had been posted onto websites. The images started with children fully dressed and the users could then 'undress' the boys - eventually being able to view images of them being sexually abused.

These images were picked up by a private organisation in Ireland, Copine, which monitors internet sites for paedophile activity.

The evidence was passed to Greater Manchester Police, a centre of excellence in this type of investigation, who traced a man called Wood, living in Thailand.

During these inquiries - which are still ongoing - it was discovered that Leach had been living in the same apartment as Wood but had returned to England last December.

Skipton police acquired a warrant and raided Leach's Cross Hills' home in January, finding 539 indecent pictures and nine movie clips of children being abused on a laptop computer.

Copine categorises such images on a scale of five levels, with level five being the most serious. The images found in Leach's possession were between level one - images depicting nudity or erotic poses with no sexual activity - and level four - pictures of penetrative sexual activity between children and adults.

The Skipton team had to classify the images, trace the children and gather the evidence to convict Leach.

"It was difficult and at times complex inquiry," DS Martin said. "Some of the images we were required to view were very unpleasant, but we couldn't get away from the fact there were 530-odd images which had to be viewed for the purposes of gathering evidence and intelligence."

Officers were seen regularly by the welfare department and were free to leave the investigation if the viewing became too much. None of the Skipton detectives did this.

"It was not something any of us wanted to do, but in order to build up our case it was a requirement of the job," he said.

Differences between the English and Thai legal system created further problems, including the classification of sexual abuse. For instance, Thailand does not recognise an offence of male on male rape.

Leach had to be charged with offences that existed both in England and Thailand. He appeared at Bradford Crown Court on 28 charges to which he pleaded guilty.

In police interview he had told officers that he had become involved with Wood after meeting him over the internet.

The court heard how he had received pornography from Wood and then went with him to Milton Keynes, where an eight-year-old boy was abused.

In October last year they both went to live in Thailand and several children - including a boy aged seven and another aged 11 - were invited into their home and abused.

DS Martin said that the men befriended street kids, buying them drinks and ice-cream, and then gradually introduced them to sexual activity.

They used digital cameras to record the images and then downloaded them onto the internet. It is not thought that this was a commercial operation.

Police are still investigating the operation, which is classed as "major" and involves authorities around the world.

DS Martin paid tribute to the Skipton team.

"A lot of hard work has gone into this in the last six months. I would like to thank the team for their efforts in what has been a difficult inquiry."

He said what had surprised him was that many of those arrested had no previous convictions.

Paedophiles tend to work in small circles, locating each other through chat rooms and news groups. They communicate and send postings to each other via email or through reference to other sites.

In this case the offences were all committed against young boys between seven and 12, but paedophile sites have images of a variety of activity against children of both gender, and various ages.

Jayne Beckett, mitigating in court for Leach, said that her client was a man of previous good character.

She said that Leach had been carried along with the "depraved activity" and "a spate of truly horrible offending which comes completely out of the blue."

Judge Stephen Gullick placed Leach on a sex offenders' register for life and banned him from working with children.

"It must be made clear to you and others that it merely perpetuates, in many ways, the abuse that is done to these children. If you indulge in this sort of behaviour, only a prison sentence will follow," he said.

o Operation Ore is continuing. One person has been convicted, another cautioned, and investigations are ongoing in several other cases.