Richard Watson: The unstable are ripe for radicalisation

The suspected grooming of a mentally impaired young man to carry out a crude terrorist attack on a family restaurant in Exeter last week has rightly been condemned as a new low in the history of Islamist terrorism in Britain. It will not, however, come as a surprise to MI5, nor to those who follow the evolution of the terror threat.

Devon and Cornwall Police's deputy chief constable, Tony Melville, has already said that the alleged bomber, 22-year-old Nicky Reilly, who lives with his mother in Plymouth, appeared to be a "vulnerable" individual who had been "preyed upon" and "radicalised". Sources close to the investigation have suggested that Reilly, who neighbours say converted to Islam last year, suffers from Asperger's syndrome, a form of autism which has apparently left him with few social skills. He is described as simple and easily led – perfect, then, to be exploited as a self-detonating mule.

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Where Iraq leads, Britain follows, it seems. Just as vehicle-borne gas cylinder bombs were used last summer at Glasgow airport – a technique imported from the streets of Baghdad – the exploitation of mentally ill people has also been honed in the Iraqi crucible of terror.

On 1 February, a woman strapped with explosives walked into the Ghazil animal market in Baghdad, killing at least 42 people. Twenty minutes later a second female suicide bomber entered another market. Her bomb killed 27.

Iraqi security forces spokesman Brigadier al-Mousawi told the BBC that "the operation was carried out by two booby-trapped mentally disabled women". He said the bombs were detonated remotely. A spokesman for the US military said that the two women had undergone treatment for depression and schizophrenia and "had a history of psychiatric treatment".

The apparent similarities between the double suicide strikes in Iraq and the approach favoured in Exeter this week does not mean this latest British attack was planned from overseas or had the imprimatur of al-Qa'ida central; that is unlikely for such a crude device and a poorly executed plan. It is likely to be a locally inspired plot, with Nicky Reilly used as cannon fodder. However, one source of inspiration is clear. The terrorism analyst Sajjan Gohel called this "strategic blowback from Iraq". He said, "As the police crack down on cells in the UK, what better strategy than to resort to individuals who are not seen as threats, people who are viewed as innocent?"

Nicky Reilly, who changed his name to Mohammed Rasheed last year, is reported to have received a text message of encouragement shortly before the attack. The investigation in the West Country will now focus on his supporters and accomplices. Arrests were made on Friday when armed police raided a café.

For terrorist recruiters, psychologically damaged or isolated converts to Islam offer twin benefits. First, they are far more likely to be drawn into a network if that network offers them a sense of purpose and respect. Second, once they have become members, they are far more suggestible.

If early reports are correct, then Nicky Reilly appears to fit the bill. One of his neighbours, Scott Allen, 19, told reporters: "He had always been a follower and had always wanted friends." He said that people made fun of Reilly who always seemed "very down".

When I was investigating the activities of Al-Muhajiroun, the Islamist group set up by Omar Bakri Mohammed, I witnessed this strategy. It was 2004 and I was making a film for Newsnight about a British convert to Islam called Simon Keeler who, incidentally, has recently been convicted of terrorism offences.

I turned up with my cameraman at a community centre in Bethnal Green in London to film a meeting which Keeler was attending. The audience of 60 Muslims listened to Al-Muhajiroun's leaders provocatively describing the 9/11 attacks as "magnificent" and urging them to swear allegiance to Osama bin Laden over Tony Blair. The chairman then introduced a dishevelled-looking man in his twenties, the group's latest recruit. Wayne Derby, a white Briton wearing a brown robe and a skullcap, was about to convert to Islam. He came to the front of the hall and started mumbling as we filmed.

"I bear witness that there is none worthy of worship except Allah and Mohamed is the Messenger of Allah." He stumbled through the lines that brought him into the faith. The leader took the microphone. "And Wayne has now become Osama!" he announced as cries of "Allahu Akbar" echoed around the room.

Osama Derby, the convert, was clearly a troubled man who had been struggling with alcohol addiction before he decided to become a Muslim. He told me: "Going back before I decided to convert to Islam, my life wasn't any sort of life. I was drinking alcohol, lack of work, lack of family around me, didn't have no family. Now I've got one billion – so many brothers around me. I couldn't ask for a bigger family. You've got all these governments doing wars and doing things and like, they're actually saying, like, they're fighting, like, Bin Laden and that saying he's a terrorist. But I look at it as Tony Blair and Bush being terrorists."

I found it interesting that Osama Derby spoke to me about the one billion brothers he now had. New recruits feel a sense of belonging and purpose which they may never have experienced before. But this is just the start. Next they are encouraged to abandon loyalty to the nation state and the civil law in favour of the worldwide Muslim "umma" – a crucial psychological break. Then a diet of horrific images of civilians being killed in coalition air strikes is used to inculcate a sense of fury which is magnified by extreme interpretations of the Koran which purport to show how Mohamed was ruthless with the infidel. This is potent stuff. In the hands of skilful propagandists, new converts can become much more extreme than those born into Islam who can draw upon moderate influences such as family.

Even when extremist recruiters target those born into Islam, they will often pick off those isolated from home. Two years ago I met Jawad, a student who had recently left Brunel University in west London. He told me he had been targeted by the extreme political group Hizb ut-Tahrir which was very active on the campus. Alone in Britain for the first time, with no family or friends to turn to, he says the network taught him to reject everything about democracy and the British way of life. "They promoted hatred towards wider UK society, towards the West, and that is how I developed my understanding about the West which at the time was very negative. I was advised not to mix with other faith communities."

There is clearly a difference between joining an extreme political Islamist group and joining a terrorist cell. But all Islamist terrorists were once extreme political Islamists who advocated the supremacy of Islam – there is a pathway to be followed.

In Plymouth, if it turns out to be true that Nicky Reilly suffers from profound socialisation problems, then the authoritative Diagnostic and Statistical Manual of Mental Disorders used by the American Psychiatrist Association may prove valuable in this case: the "marked impairment in social interaction", the "failure to develop peer relationships" and the "lack of social or emotional reciprocity" which are used to help to define autism are all factors that can place terrorist recruitment on the fast track.

Richard Watson is a correspondent for BBC 'Newsnight', specialising in terrorism and radicalisation. Click here to read Richard Watson on extremism