By Gordon Corera
BBC security correspondent
It is rare to get an insider's perspective on the emergence of al-Qaeda. It is also rare to get a glimpse of the world of spies and agents. To provide both is incredibly unusual.
Omar Nasiri - not his real name but one chosen to protect his identity - says he spent seven years working as an agent for European intelligence services and as an al-Qaeda operative, part of the time in the UK.
He provides a unique insight into how al-Qaeda was far more organised, coherent and determined in the 1990s than was appreciated at the time.
Jihad militants are trained in combat
Nasiri's story begins in the mid 1990s in Belgium after his brother became involved with a group of Algerian Islamic activists.
To make money, Nasiri began to supply the cell with guns. But after stealing some cash from the cell, he realised his life was in danger.
He knew the French were taking the threat from the Algerian Islamists seriously and so he walked into the French consulate in Brussels.
He met an officer from the DGSE, France's overseas intelligence service. Nasiri remembers that the officer offered him money - and protection - but at a price.
"He said, 'To give you what you ask, a new identity, a future, you need to do more.'" That meant becoming a spy.
Nasiri's family's house was raided in March 1995. The material discovered in that raid proved to be one of the first signs of emerging links between jihadist groups - and of the role of al-Qaeda, according to Belgian police.
As well as weapons, a training manual was found. The first page of it was dedicated to Osama bin Laden.
Meanwhile, the French had begun to hear of training camps in Afghanistan. They wanted Nasiri to investigate.
"My mission [was] to find the route of jihad through Pakistan and Afghanistan... No leads, no names, no address, Nothing. Just go find the route of jihad."
Through a series of contacts he found his way to Peshawar where he met Abu Zubaydah, the gatekeeper of the Afghan training camps who would be captured soon after 9/11 and was recently transferred from a secret CIA prison to Guantanamo Bay.
His first stop was Khalden, one of al-Qaeda's key training camps. Amongst those who attended were Mohammed Atta - the ringleader of the 9-11 attacks - and Richard Reid, the so called "shoe bomber" who tried to detonate explosives on a transatlantic flight.
Nasiri described how recruits were provided with intense and highly comprehensive military training - much of which was based on training manuals of UK and US special forces and also involved assassination and kidnapping techniques.
Spiritual preparation was also an important part of the training, taking up as much, if not more of the time.
Intelligence experts believe he provides a convincing and unusually detailed portrait of camp life.
"It's an extraordinarily complete story, and certainly I don't think there's anything to match it in terms of intelligence reporting," said Mike Scheuer, who was head of the CIA's Bin Laden unit between 1996 and 1999.
Recruits were also trained how to resist interrogation and provide false information - Nasiri's mentor at the camps, Ibn Sheikh al-Libi, would go on to provide false evidence of links between al-Qaeda and Iraq after he was captured by the US.
This evidence was cited by both the US Secretary of State Colin Powell and President Bush in the build-up to the war with Iraq.
From Khalden, Nasiri was sent to Darunta, the "graduate" school which focused on training individuals for operations.
There, recruits learnt how to make explosives and detonators from scratch.
Nasiri also witnessed chemical weapons experiments - including the use of gases and poisons on rabbits, evidence of an organised WMD programme far earlier than had previously been reported.
With his training complete, Abu Zubaydah despatched Nasiri back to Europe with instructions to set up a "sleeper" cell and to remain in contact.
Abu Hamza was a focus for the security services
"He asked me to go back... and begin to make a list of all targets," recalls Nasiri.
After resuming contact with his French handlers, Nasiri was sent to London. The French were worried that Islamic radicals were using London as a base. They called the British capital Londonistan.
Nasiri was now run jointly by French and British intelligence. He began to infiltrate Finsbury Park Mosque and spy on its imam, Abu Hamza, as well as another radical, Abu Qatada.
He passed messages back between the training camps and London, from those around Abu Zubaydah to Abu Qatada, with the British Security Service and the French listening in, he says.
He was even sent a notebook he had compiled on how to make explosives. He was told by his handlers to focus on Abu Hamza rather than Abu Qatada, a decision he disagreed with.
British police and security officials were focused on whether Islamic radicals posed a threat to the UK but not necessarily on the larger international picture and the connections going back and forth.
"At the time we didn't think that the growing threat from al-Qaeda and Osama Bin Laden was sufficient to put more resources on it," says Bob Milton, who worked on the issue at the Metropolitan Police Special Branch between 1996 and 1998.
"We were monitoring what he was doing, certainly working with the US and European colleagues to do that. But at that time we were still unsure what the threat would be."
He was left without a role just as the threat was growing
In 1998, al-Qaeda's war against the west began in earnest as US embassies in Kenya and Tanzania were bombed.
But Nasiri's relationship with the intelligence services deteriorated rapidly. He felt they were not listening to him. They may not have trusted him and have been unsure of where his loyalties really lay.
He was left without a role just as the threat was growing. But his story makes clear that al-Qaeda was far more organised, more determined and more coherent in the 1990s than anyone appreciated at the time.
Thousands - probably tens of thousands - graduated from al-Qaeda's training camps in the 1990s and the spread of ideology has left an equally lasting legacy in terms of radicalisation.