MI5's man in Al Qaida speaks out.

Dean was brought up in Saudi Arabia, where opposition to the Soviet occupation of Afghanistan in the 1980s made military jihad a noble concept. He was a teenager when Yugoslavia splintered, and Bosnian Muslims found themselves in mortal danger from Serb nationalists. He and a friend, Khalid al-Hajj - later to become the leader of al-Qaeda in Saudi Arabia - set off to become mujahideen.

I would say it was the most eye-opening experience I ever had. I was a bookish nerd from Saudi Arabia just weeks ago and then suddenly I find myself prancing up on the mountains of Bosnia holding an AK-47 feeling a sense of immense empowerment - and the feeling that I was participating in writing history rather than just watching history on the side.

Dean was at a training camp in Afghanistan when the bombings of the US embassies in Nairobi and Dar es Salaam took place in 1998. He was concerned to learn that as well as the 12 American casualties, 240 or more local people died, and 5,000 were wounded.

I think that is when the horror of it started to sink in. And this is when I realised that if this is the opening salvo of this war, where is the next target? Argentina, South Africa, Mozambique? Are we going to fight Americans in Africa in order to expel them from the Middle East, from the Arabian peninsula? It just didn't make sense.

Valued first by al-Qaeda and then British security and intelligence, Aimen Dean's life under cover came to an abrupt end when the cover was blown. An American writer disclosed his identity with details that could only be sourced to Dean. That was eight years ago.