The MI5 briefing note, Understanding Radicalisation and Violent Extremism in the UK, seen by the Guardian provides a unique insight into current thinking within the security service about how a modern-day British terrorist is made. The analysis, based on hundreds of case studies of those involved in or closely associated with terrorism, concludes that there is no single pathway to extremism. All had taken strikingly different journeys to violent extremist activity. However the security service does say that most individuals in the sample had some vulnerability in their background that made them receptive to extremist ideology. For most, radicalisation takes months or years with no one becoming a terrorist overnight, and it is always driven by contact with others. Exposure to extremist ideology, whether in the form of online communities, books, or DVDs, although crucial, is never enough on its own. Personal interaction is essential, in most cases, to draw individuals into violent extremist networks. Alan Travis reports.