The Boarding School Bomber (2011)

This film starring Adam Deacon (Adulthood, Kidulthood) sets out to answer this very question. It plays alongside sensitive interviews with Andrew's friends, classmates and his mother. Police testimony of the race to find the plotter is cut against unprecedented CCTV footage that tracks his every move through the city. Most sinister, however, is the film's portrayal of the world of online extremism which turned Andrew into a terrorist, and the actual footage he viewed online is woven through the film in stark uncut form, surely leaving every mother wondering what her son is up to behind closed doors.