...Salas decided to go undercover with his hidden cameras after the bombings that killed 191 people on Madrid commuter trains on 11 March 2004. He had been as stunned as other Spaniards by the blasts, despite the country's experience of Basque terrorist group Eta. "I wanted to know what goes through the mind of a person who is capable of killing for an ideology."

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. "My aim was to understand terrorism in the same way that I came to understand skinheads or prostitute-traffickers."

...Salas picked the Venezuela of President Hugo Chávez as his base. "I had been told Venezuela was a mecca of international terrorism," he says. "The Farc group from Colombia was there, as were people from Eta." Numerous other small revolutionary groups had also set up under Chávez's benevolent gaze. There, in what the New Yorker journalist Jon Lee Anderson calls "the parallel reality that is the Bolivarian Republic of Venezuela today", Salas established himself as yet another niche radical – flying the flag for Palestine and running a local branch of Hezbollah. More importantly, he got close to the family of Ilich Ramírez Sánchez – Carlos the Jackal.

...By tracking the Arabic news channel al-Jazeera, Venezuelan TV and the internet for mentions of The Jackal, Salas discovered that Chávez himself was one of his biggest fans. "For him, Carlos is not a terrorist but a revolutionary – a model internationalist, like Che Guevara. Just as Che went to fight for other peoples, so Ilich went to fight for the Palestinians. Whenever Chávez mentioned The Jackal, I would record it and send it to him, which he loved."

Not that Salas agrees with Chávez's view of The Jackal. "He is considered responsible for 82 killings; I don't call that being a revolutionary. I call him a terrorist." – though he would probably not, he admits, use the term to his face. "It helps that he is in jail."

...In Venezuela's fringe community of political extremists, he bumped into people from Eta, the Túpac Amaru (a group of armed Venezuelan radicals who support Chávez), and other groups. Repeated requests for hands-on training eventually saw him invited to a camp in Venezuela, where he learned to handle pistols, rifles and machine guns, including a Kalashnikov AK-103, an Uzi sub-machine gun, the American M4 carbine and a Belgian-designed FN FAL. He also practised with a sniper's telescopic sight and received explosives training. "I learned all that a jihadist might need to take his message of terror to a city in Europe or the United States," Salas says. "There was nothing glamorous about it. It was just a question of learning to kill better."

His instructors included a Venezuelan army colonel, though Salas insists the camp was not run by the Chávez regime. "It just so happened that my instructors, as well as being supporters of revolutionary causes, were Venezuelan army officers."

...So what conclusions does Salas draw from rubbing shoulders with international terror? His answer is coloured by the fact that half a dozen people he met during his investigation have since died – often violently. "I don't justify violence, but I can understand it. I never found any glamour or sophistication in that world, nor anyone especially intelligent – except for The Jackal. Terrorists really have only two ends – they either die or go to jail. You have to be a bit stupid to do that."