In a greasy yard in Tabqa, underneath a date palm, three boys are busy at work rebuilding a lorry engine. They are covered in motor oil. Their hair, black and oily, stands on end. Near them is a group of drivers. Abu Fawzi is at the centre, conspicuous in his bright red jacket. It matches the colour of his beloved 18-wheeler. He’s clearly the leader, quick to offer tea and cigarettes. At first he says he doesn’t want to speak but soon changes his mind. He and the rest of the drivers are angry. It’s weeks since they risked their lives for a journey that ruined engines and broke axles but still they haven’t been paid. It was a journey to hell and back, he says.

One of the drivers maps out the route of the convoy

“We were scared from the moment we entered Raqqa,” he says. “We were supposed to go in with the SDF, but we went alone. As soon as we entered, we saw IS fighters with their weapons and suicide belts on. They booby-trapped our trucks. If something were to go wrong in the deal, they would bomb the entire convoy. Even their children and women had suicide belts on.” The Kurdish-led SDF cleared Raqqa of media. Islamic State’s escape from its base would not be televised. 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 - our vehicle and their vehicles combined. When we entered Raqqa, we thought there were 200 people to collect. In my vehicle alone, I took 112 people.”

Another driver says the convoy was six to seven kilometres long. It included almost 50 trucks, 13 buses and more than 100 of the Islamic State group’s own vehicles. IS fighters, their faces covered, sat defiantly on top of some of the vehicles. Footage secretly filmed and passed to us shows lorries towing trailers crammed with armed men. Despite an agreement to take only personal weapons, IS fighters took everything they could carry. Ten trucks were loaded with weapons and ammunition.

The drivers point to a white truck being worked on in the corner of the yard. “Its axle was broken because of the weight of the ammo,” says Abu Fawzi. This wasn’t so much an evacuation - it was the exodus of so-called Islamic State. The SDF didn’t want the retreat from Raqqa to look like an escape to victory. No flags or banners would be allowed to be flown from the convoy as it left the city, the deal stipulated. It was also understood that no foreigners would be allowed to leave Raqqa alive. Back in May, US Defence Secretary James Mattis described the fight against IS as a war of “annihilation”.“Our intention is that the foreign fighters do not survive the fight to return home to north Africa, to Europe, to America, to Asia, to Africa. We are not going to allow them to do so,” he said on US television. But foreign fighters – those not from Syria and Iraq - were also able to join the convoy, according to the drivers. One explains:

There was a huge number of foreigners. France, Turkey, Azerbaijan, Pakistan, Yemen, Saudi, China, Tunisia, Egypt...”

Other drivers chipped in with the names of different nationalities. 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. “We didn’t want anyone to leave,” says Col Ryan Dillon, spokesman for Operation Inherent Resolve, the Western coalition against IS. “But this goes to the heart of our strategy, ‘by, with and through’ local leaders on the ground. It comes down to Syrians – they are the ones fighting and dying, they get to make the decisions regarding operations,” he says. While a Western officer was present for the negotiations, they didn’t take an “active part” in the discussions. Col Dillon maintains, though, that only four foreign fighters left and they are now in SDF custody.

IS family members prepare to leave