Lorry driver Abu Fawzi thought it was going to be just another job.
He drives an 18-wheeler across some of the most dangerous territory in northern Syria. Bombed-out bridges, deep desert sand, even government forces and so-called Islamic State fighters don’t stand in the way of a delivery.
But this time, his load was to be human cargo. The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters opposed to IS, wanted him to lead a convoy that would take hundreds of families displaced by fighting from the town of Tabqa on the Euphrates river to a camp further north.
The job would take six hours, maximum – or at least that’s what he was told.
But when he and his fellow drivers assembled their convoy early on 12 October, they realised they had been lied to.
Instead, it would take three days of hard driving, carrying a deadly cargo – hundreds of IS fighters, their families and tonnes of weapons and ammunition.
Abu Fawzi and dozens of other drivers were promised thousands of dollars for the task but it had to remain secret.
The deal to let IS fighters escape from Raqqa – de facto capital of their self-declared caliphate – had been arranged by local officials. It came after four months of fighting that left the city obliterated and almost devoid of people. It would spare lives and bring fighting to an end. The lives of the Arab, Kurdish and other fighters opposing IS would be spared.
But it also enabled many hundreds of IS fighters to escape from the city. At the time, neither the US and British-led coalition, nor the SDF, which it backs, wanted to admit their part.
Has the pact, which stood as Raqqa’s dirty secret, unleashed a threat to the outside world – one that has enabled militants to spread far and wide across Syria and beyond?
Great pains were taken to hide it from the world. But the BBC has spoken to dozens of people who were either on the convoy, or observed it, and to the men who negotiated the deal.