As I understand it, Al Qaeda is not organised as heirarchically as you might think.
Various experts in the serious newspapers say that its much more like a franchise system, there isn’t any obvous way to rise up within the organisation, more like small units are recruited by perhaps a group of like minded individuals, who all know each other very well.
They then start looking around for things to do, but in effect they are almost their own little stand alone terrorist group.
Quite how they then go on to acquire materials and support, I do not know, but it appears to be extremely decentralised.
Infiltration into a group who may have known each other since childhood is likely to be extremely difficult, its a problem that law enforcement agencies have with neighbourhood drug dealers too and this makes catching those further up the chain very difficult.
It seems intuitive to me, that these individuals must somehow be schooled in intelligence matters, ‘need to know’ and all that stuff, no doubt other resources such as money and technical knowledge also must be distributed, and that this would be a means to track them.
There will be all sorts of methods used, from tax records, through to accidental things like being involved in unpredictable incidents that happen to us all such as traffic mishaps, mobile phones, chatrooms etc.
Frankly I would be amazed and very dissappointed if our intelligence services didn’t employ Arabs, from individuals right through to agencies operated by other Arab nations.
I guess we will never know how many will simply just ‘dissappear’, of the plots that never came to fruition, the worry is that we have to get it right 100%, they only need get it right once.