If you didn't see the recent BBC2 'conspiracy-debunking' programme about the four years ago 7/7 attacks in London , and if you'd like to know why some people don't believe the official version , it's summarised very well here :
http://www.dailymail.co.uk/news/article-1197419/Conspiracy-fever-As-rumours-swell-government-staged-7-7-victims-relatives-proper-inquiry.html