It was a defining moment, the first time 'Jihadi John' appeared. Suddenly Islamic State had a face and the whole world knew the extent of their savagery. Weeks later, when his identity was revealed, Robert Verkaik was shocked to realise that this was a man he'd interviewed years earlier.

Back in 2010, Mohammed Emwazi was a twenty-one-year-old IT graduate who claimed the security services were ruining his life. They had repeatedly approached him, his family and his fiancée. Had they been tracking an already dangerous extremist or did they push him over the edge?

In the aftermath of the US air strike that killed Emwazi in November 2015, Verkaik's investigation leads him to deeply troubling questions. What led Emwazi to come to him for help in the first place? And why do hundreds of Britons want to join Islamic State? In an investigation both frightening and urgent, Verkaik goes beyond the making of one terrorist to examine the radicalisation of our youth and to ask what we can do to stop it happening in future.