Security Service 101. (Pay attention, James...!)
...Now maybe NSA can get some better examples, but is suspect that their methodology only works after the perps are already known....
This is all a giant budget-justifying exercise after the loss of the Russians as major justifiers of funding when the Cold War ended.
For those of you who seem poorly informed, a typical Security Service MO follows this path...
1 - pick up someone a little dodgy in a Muslim area by asking the local police for names - perhaps get them picked up for a minor crime.
2 - 'persuade' them to act as an 'agent'. Forgetting the charge is very helpful here.
3 - get them to report on anyone they know who is discussing terrorism. This will, of course, pick up all sorts of people, including people that the agent lied about in order to have something to write down.
4 - bug their phones and watch them. This is where the justification for GCHQ and PRISM comes from. Depending on what people say when they're drunk, you can soon have lots of low-grade evidence that there is a terrorist threat of any level you want.
5 - If you need an arrest, get your agent to encourage some of the stupidest hot-heads to think that they could be Jihadis. Provide them with some incriminating literature. Suggest that you can find a bomb for them to put under a bridge...
6 - After delivering a dummy bomb/lots of castor oil seeds/WHY, arrest them in a dawn raid with armed police and full press coverage. Charge them with being a sleeper cell for Al-Queda and have them locked up forever.
7 - start again by bugging their friend's phones. Some of their friends should be really pissed off, and might well be encouraged to be the next on your list...