Lorry driver Abu Fawzi thought it was going to be just another job. He drives an 18-wheeler ... in northern Syria. 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 ... to a camp further north. [He was told] the job would take six hours. He and his fellow drivers ... 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. The deal to let IS fighters escape from Raqqa ... would spare lives and bring fighting to an end. 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. Publicly, the SDF said that only a few dozen fighters had been able to leave, all of them locals. But one lorry driver tells us that isn't true. We took out around 4,000 people including women and children," [the lorry driver said]. The convoy was six to seven kilometres long. In light of the BBC investigation, the coalition now admits the part it played in the deal. Some 250 IS fighters were allowed to leave Raqqa, with 3,500 of their family members.