This Canadian parliamentary report has some depth - in the variety of witnesses heard - and the testimony of Professor Andrew Silke (UK) on the effect of prison is important:http://parl.gc.ca/40/3/parlbus/commb...df/09issue.pdf

Perhaps another surprising observation is that in the U.K. we have had over 400 al Qaeda-linked extremists convicted of terrorism-related offences since 2001. Approximately 300 of these people have been released already, and many have received relatively short sentences. Most of these people are on the streets in the U.K. One finding is that there does not seem to be any evidence of these people re-engaging in extremism or becoming involved in violence again. The re-conviction rate or the re-offending rate of these individuals is extremely low. It has surprised many people who assumed that if someone was radicalized, chiefly engaged in violence and had spent time in
prison, they would still be dangerous when they came out. From most of them we see that prison represents a transition period where they move on to other issues and away from violence.
Link to report:http://www.parl.gc.ca/40/3/parlbus/c...p03mar11-e.pdf