By Nasreen Suleaman
Why did he do it? Months of investigations have uncovered an apparently integrated, happy, western-thinking man behind the face of one of Britain's first suicide bombers. But many questions remain unanswered.
Like millions of others, Ian Barrett watched the news just a week after the 7 July bombings to discover that three of the men responsible were from the Beeston district of Leeds.
But it was only later that evening, when old school friend Rob Cardiss called Ian on his mobile, that he realised he knew one of the bombers.
"Do you realise who Mohammed Sidique Khan is?" asked Rob.
Ian didn't recognise the name. Rob told him: "It's Sid - from school!"
Mohammed Sidique Khan was someone Ian had lost touch with, but at school they had been close friends. Khan's story is not one of cultural isolation, racial segregation or adolescent religious indoctrination.
The Beeston of Khan's youth was a largely white neighbourhood - and indeed he seems to have spent most of his time in the company of white English lads. Over the past few months I have spoken to many of those white childhood friends, friends who knew Khan as Sid, and they all tell a similar story.
Their accounts of Khan's upbringing and character show a man who spent most of his formative years not really mixing with other local Muslims.
And, says Ian Barrett, unlike the other children of Pakistani parents, he was not under any family pressure to take an interest in Islam.
Mohammed Sidique Khan in London two weeks before the bombings
"The other Pakistani lads would have to go mosque because their families would say 'You're going to mosque.' But Sid didn't go," says Ian. "He didn't seem interested in Islam and I don't ever remember him mentioning religion."
Khan was, by all accounts, an exceptionally well integrated person. His anglicised name "Sid" was just one symbol of his willingness to take on a British identity.
"If it wasn't for the colour of his skin, he would have been [seen as exclusively] English," says Ian. "I just thought of him as a Beeston lad - and that's what he was - a Beeston lad, born and bred."
During the 1970s and 80s, the Muslim population of Beeston swelled. But while the community grew in number and confidence, Khan appears to have negotiated the potential for divided loyalties in a multicultural society with remarkable social skill.
Ian Barrett and Rob Cardiss recall how fights would regularly break out between English and Asian lads at their secondary school.
Beeston: Fears prompted silence, amid media storm
Khan never took part - and somehow also managed to avoid being reprimanded, by either side, for remaining neutral. In fact, the only criticism he appears ever to have attracted was some mild adolescent teasing from the other Asian lads about his friendship with an English female classmate.
So how did he become one of Britain's first suicide bombers?
With a decade of experience in journalism, fluent Urdu and a Yorkshire upbringing, I assumed I was well placed to discover what led to Khan's remarkable transformation.
But no one could have prepared me for the febrile atmosphere and wall of silence that has been built up by the Beeston Muslim community that knew him. What is clear is that many people are either too scared to talk - or scared that if they do, that what they say will be distorted by the media.
When the world's press first arrived - and I mean scores of journalists - we were treated with courtesy and respect by Beeston's Muslim community.
We immediately descended on Cross Flatts Park, close to the streets where Khan grew up and where he and the other bombers had played sport.
The local Pakistani lads were heartbroken to learn that Khan could have been responsible for killing people. They were willing to reminisce for the cameras about how Sid was a decent and popular guy.
However as the press began to report stories about radicalisation taking place in local gyms, youth centres and Islamic bookshops, the Muslim residents of Beeston became angry and then wary.
We have heard the second-hand stories, the rumours and the speculation. But we have yet to hear the first-hand testimony of those who attended these places to really know if any kind of radical Islamist ideology was being spread by Khan or others.
When I returned to Beeston several weeks after the attacks, the silence had been partially lifted.
But there was another barrier to getting at the truth: the willingness of many people to prefer conspiracy theories to some honest reflection about how three young men in their midst could have carried out these terrible attacks.
I was told frequently that the 7 July bombers were either duped into it or were innocent victims of somebody else's bombing campaign.
One Muslim young professional spoke for many when he told me that if the Metropolitan Police could have shot Jean Charles de Menezes, an innocent young Brazilian mistaken for a potential suicide bomber, then they could also be wrong about Khan.
Pakistan: Khan's airport security shot in Karachi, February 2005
I told him that if he had any evidence that undermined Khan's guilt then that would be a sensational story indeed.
The release of Khan's suicide video has diminished some of the doubts about his role in the attacks. But to some extent those doubts still persist in Muslim areas - not only in Beeston but among many other British Muslims I have spoken to.
That's not to say that since July 7 Muslims haven't been asking some important questions. The Muslims I speak to want to know how Khan, a British Muslim like them, did what he did.
We have discovered that not only, as we suspected, there is "an enemy within" - but that its nature is highly complex. Mohammed Sidique Khan exemplifies that complexity.
Here was a Muslim who was publicly respected and admired. He was neither socially isolated nor economically disadvantaged.
If he, with all his trappings of Western culture, is capable of this, how can we prevent it happening again?
And, most uncomfortably for those of Muslim origin like myself, does it encourage our non-Muslim neighbours to look on us all with suspicion?
Biography of a Bomber will be broadcast on BBC Radio Four on 17 November.
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