Tunisia attack: Why did Seifeddine Rezgui turn from student to killer?

Seifeddine Rezgui, 23, was studying electronics when he joined Islamic State and turned a gun on a beach full of tourists in Sousse, Tunisia, leaving 38 people dead.

But what makes a student become a mass murderer?

Special correspondent Fergal Keane travelled to Rezgui's hometown of Gaafour to meet his friends and family.