The final part of his cover was to become a pro-jihad journalist, contributing to radical publications. 'Not many people carry their own, hand-copied version.' He treasures the small booklet in which he wrote Islam's most sacred text: 'It helped convince people,' he says. We mocked up an apartment in Barcelona to look as though it was in Palestine and took photos.' Salas also wrote out the Qur'an by hand, and considers his conversion to Islam to be genuine. Then I asked Fatima, a girl I met when investigating prostitute-trafficking, to let me take photos with her as if she was my wife. 'I took photos of myself in Burkin and in Jenin. Her death, he would claim, had pushed him towards radical terror. In case anyone decided to investigate, he added a Romeo and Juliet touch: the marriage had been kept secret because his (false) mother's family, from the nearby village of Burkin, backed Al-Fatah, while Dalal's family were part of Hamas. The real Dalal, whose name he found in a newspaper archive, had died in 2004, when a bullet entered her house in a shoot-out.

He learned Arabic and invented an elaborate cover story involving a dead wife: 25-year-old Dalal Mujahad from Jenin, tragically killed by an Israeli bullet while pregnant with their child.

'My aim was to understand terrorism in the same way that I came to understand skinheads or prostitute-traffickers.' Salas's previous undercover investigations – as a skinhead supporter of Real Madrid football club, and in the world of prostitute-trafficking – had taken him to the heart of some of the most violent groups in Spain.

'I wanted to know what goes through the mind of a person who is capable of killing for an ideology.' He had been as stunned as other Spaniards by the blasts, despite the country's experience of Basque terrorist group Eta. Salas decided to go undercover with his hidden cameras after the bombings that killed 191 people on Madrid commuter trains on 11 March 2004. 'It is a strange sensation when a self-confessed assassin like Carlos the Jackal does that, and offers their friendship.' 'He was very worried about my security,' says Salas. The Jackal would call from La Santé prison in Paris, where he is still serving a life sentence for murder.

The jackal jr ward