When US-backed Syrian fighters took full control of the city of Raqqa, it ended three years of rule there by so-called Islamic State. But now the BBC has uncovered details of a secret deal that let several hundred IS fighters escape. IS made Raqqa in northern Syria its headquarters in early 2014. Last month Raqqa fell, but this programme has learnt that in exchange for a deal to save lives and bring peace to the city, a convoy carrying several hundred IS fighters, their families and weapons and ammunition -- were able to leave the city freely. The question now is, where are they now? Our Middle East Correspondent, Quentin Sommerville, has this exclusive report.