When al-Qaeda attacked the twin towers, I was in a BBC editing suite, putting the final touches to a television series on the "Hairies" - the Special Branch officers who infiltrated "subversive" organisations. My feelings were those of incredulity, followed by horror. I realised that the focus of my work would have to change. I'd spent much of the previous two decades trying to explain the Irish conflict. I never imagined I would spend the next ten years trying to do the same with al-Qaeda and Islamist extremism.
I had tracked al-Qaeda from a distance and, almost 20 years earlier, had made a prophetic BBC documentary, Men of God, about the devastating attacks by Islamist suicide bombers on the US embassy and Marine Corps base in Lebanon in 1983 - attacks that al-Qaeda was to replicate in the years to come. Yet I still felt woefully ignorant on 11 September 2001, just as I did about Ireland and the IRA when I first covered Bloody Sunday in 1972. I knew that I needed to learn more about al-Qaeda and Osama Bin Laden and try to understand why 19 young Muslims were prepared to carry out such an attack and become "martyrs". The question kept recurring in the years that followed and was highlighted by the four young British Muslims who bombed London on 7 July 2005. Two of them answered it in their suicide videos. Iraq. Afghanistan. Palestine. Bush and Blair.
Trying to reflect on television the deeper reasons for radicalisation - alienation, discrimination and generational differences, all exploited by extremist Islam - proved to be enormously difficult. I found many Muslims reluctant or actively hostile to engaging with these questions. Conspiracy theories around 9/11 and 7/7 were everywhere. I felt that I was trying to understand a community in denial. The real conspiracy was that of silence. Many Muslim clerics were complicit by refusing to condemn atrocities publicly. It sometimes felt as if I was part of the problem; the media - television, in particular - were seen as the enemy. I lost count of the number of doors that were closed in my face. It brought home to me how limited the medium is when it comes to dealing with such emotional and sensitive complexities.
When I was making my series Generation Jihad, I wanted to interview two young Muslims who had just been released from jail after doing time for terrorist offences. They were prepared to talk but not to me and finally agreed to be interviewed by a young, Muslim colleague. What they had to say was more important than to whom they said it.
What I did see on those rare occasions when I was able to break down the barriers was the devastation caused to families by sons who had embraced jihad. Loving parents had become traumatised, often incredulous that their children would contemplate killing others and themselves. Although I met several such families, few were prepared to talk publicly, afraid of being stigmatised in their community and stereotyped as terrorists by some of the media. Those who were prepared to speak out, sometimes tearfully, were courageous and only did so to try to help other families and their children avoid meeting the same fate.
The subject that I regret coming to so late in the day is torture. Though I had investigated this area in the earlier years of the conflict in Northern Ireland, it was only when I read the series of top-secret memoranda between the US department of justice and CIA legal advisers that I realised the true significance of the "enhanced interrogation techniques" (EITs) that the Bush administration had authorised. The cold print describing how waterboarding should be conducted is chilling. How can this, applied to Khalid Sheikh Mohammed 183 times, not be torture? Eliza Manningham-Buller, former director of MI5, in a rare interview for my series The Secret War on Terror, was unequivocal in her condemnation. She had no hesitation in describing waterboarding as "torture" and regarded it as a "propaganda coup" for al-Qaeda.
The killing of Bin Laden also made me wonder, as I later wrote in an update of the programme, whether the intelligence trail that led to his hideout in Abbottabad began with the application of the EITs, thus posing the question: "Did the end justify the means?" And what about alleged British complicity in torture? When I interviewed Pervez Musharraf, former president of Pakistan, he seemed to have little doubt. He denied that the UK government had ever informed him that they didn't want Pakistan to ill-treat suspects that the British might want to talk to. "Maybe they wanted us to carry on whatever we were doing," he said. "It was a tacit approval of whatever we were doing." Manningham-Buller emphatically denied this was the case. I think the verdict on complicity is still out and questions remain about the independence of Peter Gibson's forthcoming inquiry, given its boycott by solicitors acting on behalf of some of the alleged victims of torture.
I've learned much in the ten years since the attacks of 11 September 2001 but, as with Ireland, there is so much more to understand - a vital process, if we are to combat the threat.
Peter Taylor is a BBC reporter and the author of "Talking to Terrorists: Face to Face With the Enemy" (HarperCollins, £9.99)