Jihadi John and other ISIS extremists are building a network of 'misfits' around the world who can carry out 'chaotic' terror attacks with little planning, a senior police officer has warned. Assistant Commissioner Mark Rowley, head of specialist operations at the Metropolitan Police, said ISIS is trying to 'pick up' potential fighters among 'the vulnerable, people with violent backgrounds, very young people and those with mental health issues'. Mr Rowley issued the warning of new tactics among Islamist fanatics at the Counter-Terror Expo at Olympia, west London, yesterday. London-born ISIS execution Jihadi John is being used to recruit a network of 'misfits' around the world, made up of the 'very young, violent and mentally ill', a senior police officer has warned . On changing methods of attack, Mr Rowley said: 'We still have groups who are interested in the awful, spectacular attacks that we have seen with Al Qaeda in the past. 'That's still something we have to be concerned and worried about. But now there's a much wider range, with chaotic attacks across the world using knives, using vehicles. 'The real aim being to kill and get footage of that to propagate the message and propagate their cause.' His words echo the case of London teenager Brustholm Ziamani, who was jailed in February for plotting a 'Lee Rigby-style' attack on a British soldier or policeman. He fell under the spell of the notorious al-Muhajiroun group once fronted by hate preacher Anjem Choudary after being made homeless in London. In 2008,  22-year-old Nicky Reilly - who had a mental age of 10 - attempted to carry out a suicide bombing in Exeter after he was contacted by extremists online. Assistant Commissioner Mark Rowley said yesterday that the tactics of terror groups are changing . Mr Rowley said there had been a 'step-change' in the scope and scale of terror threats. Citing the rise of ISIS in Syria and northern Iraq, compared to existing terror strongholds in Afghanistan, Somalia and Yemen, he said: 'It's a much easier area to get to. It's not far from budget holiday trips to Turkey and Greece that those terror groups travel to, to exploit.' More than 600 British jihadists have travelled to join ISIS, with half returning to pose a threat on UK streets. But Mr Rowley said around 50 per cent did not have terror links before flying to Syria. Instead, many had been radicalised via propaganda on social media, by extremists, hate preachers and fighters such as  Jihadi John - real name Mohammed Emwazi. Branding ISIS a cult, Mr Rowley added: 'It is actually looking to inspire people to support their cause. 'They're developing that with a very, very determined approach to exploiting social media and propaganda.' London teenager Brustholm Ziamani (left) was picked up by Islamic extremists after he was kicked out of his home. Mentally ill Nicky Reilly (right) was also goaded into carrying out a nail bomb attack in 2008 . Mr Rowley added: 'We have terrorist groups now able, from relatively safe environments in ungoverned space of broken areas of the country, to reach into communities in the UK and influence people to act in their name to commit terrorist act. That is a fundamental difference in the challenge the UK is facing.' The speech came amid reports that ISIS chief Abu Bakr al-Bagdadi has been injured in an air strike in Iraq and is no longer in control of the group. It is said his deputies in the organisation have been running day-to-day activities while he recovers, following the reported attack on March 18. Reilly hoped to kill or wound as many diners as he could with the crude nail bomb he had made . In a display of the tactics outlined by Mr Rowley, a man with learning difficulties was coaxed into carrying out a botched restaurant bombing in 2008. Nicky Reilly, from Plymouth, tried to blow himself up in a busy Giraffe restaurant in Exeter after accessing an extremist website online. He was preparing the nail bomb device in the restaurant's toilet when it went off in his hands, injuring him but no one else. Deputy Chief Constable Tony Melville, who investigated the case, said at the time: 'He was preyed upon, radicalised, and taken advantage of.' Reilly's mother Kim told how he had a mental age of 10 and suffered from mental illness. She said he was an 'easy target, easy prey' for radicalisation. Judge Mr Justice Calvert-Smith, who jailed Reilly for life for attempted murder, said after the trial: 'There is evidence from materials seized that he became increasingly drawn to violent action in support of fellow Muslims in revenge for perceived persecution, and to the idea of himself personally becoming a martyr.'
Met Office Assistant Commissioner says fanatics' tactics have changed .
Rather than large-scale 9/11-style attacks, smaller cells are now forming .
ISIS uses videos like those featuring Jihadi John to inspire followers .
Extremists are now targeting the mentally ill and very young, he said .