I think the problem with terrorism is it's simultaneously more and less than they think. More from the groups they don't expect, and less from the ones they expect it to come from and are surveillance and infiltrating.

For example, looking back over the history from what has been declassified in my country, the intelligence services spent a huge amount of time and resources infiltrating and surveillance communist groups and university socialist clubs, and then seemed to be completely blind-sided by the rise of Islamic terrorism when 9/11 rolled around... In a similar vein I think to how the UK is spending all this time going after people waving signs supporting Palestinians - they probably honestly think there's a real threat there, and it will turn out to be a huge waste of time and the next real terror threat will come out of some other unexpected group.

As for assault - yes, it's usually someone they know. Which is why it's ridiculous the resources they spend trying to backdoor private messaging etc. in the name of "protecting the children" when much of it's happening in person...