When Islamic State's black-masked executioner, 'Jihadi John', was revealed to be Mohammed Emwazi, a 26-year-old IT graduate from west London, senior security editor Robert Verkaik was shaken more than most.

In 2010 he'd interviewed this man.

At the time Emwazi had claimed MI5 were ruining his life. He was desperate for his story to be told, believing that going public might force the security services to leave him alone. He later told Verkaik that he felt like a 'dead man walking.'

Verkaik's investigation into the making of 'Jihadi John' leads him to the disturbing questions that Emwazi left behind.

What led him, and many other young Muslim men, to come to Verkaik for help in the first place?

And why do hundreds of other Britons wish to join Islamic State?

Frightening, thoughtprovoking and urgent, Jihadi John assesses the threat IS poses to the UK and examines how the actions of our security services might help create the same enemy we're trying to defeat.