For the next two weeks, all of the men would be woken up before dawn. They would perform the dawn prayer, then go outside for running and press-ups before the sharia lessons began at first light. The lessons were very basic, focusing on the difference between Muslims and non-Muslims, and the requirement to fight infidels and apostates.

One night the emir in charge of the training course, a bald Syrian with pale skin who, in his previous life, had been a history teacher in Homs, said there was a special event in store.

Once the men were all seated on the cave floor, the emir turned on the projector and a video flickered on the cave wall: an Arab man in an orange jumpsuit in a cage. Flames licked towards the cage, following a trail of petrol, and engulfed the man.

A voiceover intoned that this was the Jordanian pilot Moaz al-Kasasbeh, who had been captured after his plane crashed. His grotesque execution by fire, in February 2015, was seizing the world's attention at that moment, and even some jihadis were denouncing it as an immoral act.

The emir stood up and explained that this pilot had dropped bombs on Muslims, and his execution by fire was a just retribution under Islamic law. The men listened in silence.

Abu Ali soon sensed dozens of eyes turning in his direction. He was the only Jordanian there, and they all knew it. He had not said anything, but his horror at the video must have been visible on his face. The emir also stared at him. 

Depraved: Jordanian pilot Moaz al-Kasasbeh was captured and burned to death by Isis in February 2015 after his aeroplane crashed over Syria

Intense: The emir in charge of Abu Ali's Isis training played a horrifically graphic video of Jordanian pilot Moaz al-Kasasbeh, pictured above in the cage. The pilot was burned alive and the video was a test of loyalty for Abu Ali, who is also Jordanian

Silence: The horrific video of the pilot, above, which had been released worldwide, stunned the new recruits into silence in its depravity but Abu Ali could feel all eyes turning to him, because he was also Jordanian

Horror: As the video came to a close, Abu Ali heard himself shout 'May God help me' and the emir demanded to know why he had spoken those words, if he was horrified by what he had seen