It is estimated more than 100 British people could be fighting with opposition forces in Syria. At least one is known to have been killed in action earlier this year. File on 4 investigates who these men are and why they have gone to fight.While some are believed to have strong Syrian connections and are motivated by personal opposition to the government, there are concerns that others have travelled there to join hardline Islamist groups. Jenny Cuffe examines how fighters are recruited and the routes they use to join up with rebel forces. The programme hears from people who have travelled to Syria and asks what danger, if any, they may pose when they return to the UK.Producer: Paul Grant.