For some, the internet is merely a hiding place — a web of secret corridors where all manner of shameful deeds unfold. But the police never expected that it might become a strategic platform where two groups of society's outcasts, terrorists and child sex abusers, could meet to exchange operational secrets.

The realisation that there might be something in common between violent Muslim fanatics known for their supposed piety and sexual deviants who prey on children has only slowly dawned on officers. Cracking the mystery of how these worlds overlap is expected to improve understanding of the mindsets of both types of criminals and has been hailed as a potentially vital intelligence tool to undermine future terrorist plots. “A way of finding who the extremists and terrorists are”, an anti-terror source said, “is to go through the child-porn sites.”

The link might have remained unknown but for the case of a Muslim preacher from the East End of London who in 2006 was being investigated by police over his suspected links to a jihadi terrorist gunrunner.

To Scotland Yard's surprise, the 26-year-old Abdul Makim Khalisadar, a former primary school assistant, was discovered to be downloading considerable quantities of child pornography. A DNA test showed he was the wanted “Whitechapel Rapist” who had violently attacked a woman in the street a year earlier. He was jailed for ten years for rape and perverting justice. Khalisadar, who has never been convicted of terrorist offences, and some friends concocted a false alibi that he was preaching at the East London Mosque when the attack happened. He was accused of possessing photographs of child sex abuse but these 11 charges were allowed to lie on file.

Khalisadar's case came hot on the heels of the unexpected discovery of a few dozen images of hard-core child pornography during a raid on a suspected Muslim terrorist's home during a separate investigation. It was enough to convince some officers that they had discovered a potentially important link.

But an investigation by The Times has discovered that the first evidence actually came on the Continent within a few weeks of the 9/11 massacres. The unlikely setting was the Via Quaranta mosque in Milan. This place of worship was, according to the book Al-Qaeda in Europe, by the terrorism expert Lorenzo Vidino, expressly “built to create a new gathering place for militants in the southern part of the city”. It was run by the al-Qaeda recruiter Abdelkadar Mahmoud Ed Sayed.

During a crackdown on the mosque, police were astonished to discover pornography on computer hard drives. But what was not reported then was that the haul included images of children being sexually abused that were encoded with messages as a clandestine method of communication. Ed Sayed was sentenced to eight years in absentia in 2004 for terrorism-related offences.

Stefano Dambruoso, Italy's anti-terror magistrate, said: “In our experience in investigating Islamic cells linked to al-Qaeda, they use pornographic images simply to camouflage the content of their messages. They use the images — of men, women and children — as an instrument to hide messages of quite a different content.

“I would exclude the idea that they have paedophile tendencies. The most you can attribute to them is a relationship between men and women different from that of us Westerners, in which — as in many parts of the Arab world — wives are often very young girls of 11, 12 or 13 who because of family negotiations are given in marriage to men much older than them. But that is not paedophilia, it is a question of Arab culture.”

The Times has also found a case in Spain where an Islamic terror suspect is accused of downloading child pornography, a case in Yorkshire where child protection officers stumbled on a nail-bomb terror plotter, and a case in Salford where officers discovered a chemistry student visiting explosives websites and also downloading child abuse images. The Spanish case, still before the courts, resulted from raids by the Guardia Civil directed at breaking up a terrorist cell in October 2007. Thousands of hardcore pornographic images of young children were found on home computers.

Abdelkader Ayachine, an Algerian in his forties working in Burgos, is in custody awaiting trial for terrorism. He is accused of incitement to jihad via the internet by recruiting volunteers to fight in Iraq and Afghanistan, and of sending money to prisoners belonging to an Islamic terrorist movement. But he had a dual use for the internet, if another charge is proven. Prosecutors say “he regularly downloads and transfers” from the internet to the computer installed in his home “numerous video and photographic files of child pornography, in which the main characters are minors having sex among themselves or with adults”.

In police raids on him and five other suspects, investigators discovered bomb manuals and text, audio and video in which he and the other suspects were said to praise jihad. He is linked to the Islamist terrorist group that killed 45 people in a suicide bomb attack in Casablanca in 2003.

One area that British anti-terror investigators are now keen to look at is the startling similarity in the way that jihadis and paedophiles target vulnerable young people, first befriending them and then slowly introducing them to warped behaviour that comes to be seen as normal. “What we were starting to see was a similarity in grooming that goes on in paedophilia and grooming that goes on in extremism,” said the anti-terror source.

The source explained that both types of criminal also share a need for great secrecy and indeed it is the paedophiles' status as outcasts as well as their expertise in encryption techniques that may have first attracted the terrorists. Hardline Muslim recruits are often given passwords and keycodes to terrorism sites via internet chatrooms, although sometimes they come from sympathisers in local mosques. But recently British police have managed to crack some of the codes that prohibit outsiders from accessing the more hardcore jihadi sites. Using child porn sites might be one way round this.

Some paedophiles have become adept at encrypting information and burying it so deeply in the internet that no outsider can easily find it. Paedophiles then meet in cyberspace and swap notes on how to reach the images. None is likely to rush to police saying they suspect that they have spotted a terrorist loitering on their child porn website.

Another area investigators will want to explore is the similarity between the personalities of paedophiles and terrorists. “If they are going out, a lot of time is spent by going to the mosque or going off to internet cafés,” the source said.

Shahien Taj, the director of the Henna Foundation, which deals with domestic violence against women and children, said that both terrorists and paedophiles were obsessed with control and domination. She attacked the hypocrisy of terrorists who claim to espouse religious motives on one hand while degrading children on the other.

Not every terrorist downloading child pornography is a Muslim, though. The British Child Exploitation and Online Protection Centre was investigating the case of paedophile Philip Thompson, known as the Librarian because he lent out his 241,000 images of child abuse. They sent an intelligence file to police about a suspected associate, Martyn Gilleard, 31, a forklift truck driver and Hitler enthusiast from Goole on Humberside.

When officers turned up looking for child abuse images, they found 39,000 of them. But they also stumbled across Gilleard's stash of machetes, swords, bullets, gunpowder and nail bombs. He wanted to start a race war.

There is another case involving a white man that may be telling. Edward Mattison, a 21-year-old chemistry student from Salford, was jailed in 2006 after he admitted explosives offences. He created homemade bombs using chemicals including a deadly substance known as “Mother of Satan”, used in the London bombings and by Palestinian suicide bombers. He admitted offences relating to seven images of child pornography, though his lawyer said this happened just once through curiosity. He was never accused of terrorism.

Through glimpses of these characters, a pattern can be seen: the same kind of obsessive, sometimes paranoid, individual who becomes skilled in locating the rotten fruit of the internet, from bombmaking instructions to child pornography.

Just as the paedophiles have been getting cleverer at hiding their abuse images, the authorities have been raising their technological game. But it is feared that clues to terror plots may have escaped police attention because of a lack of communication between Scotland Yard's child protection and anti-terrorism specialists.

“It's worth researching this [link between terrorism and child pornography] further because we will get an operational strategy with the paedophile unit when they are infiltrating a paedophile site,” the source said.

“If we are now seeing that they are using these kind of sites as a smokescreen, as a safe haven, they will never think we are cops.”

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