Neither. Successful anti-terrorism is about nipping this stuff in the bud before the potential perpetrators get close to having specific plans. You can easily think of tactics that would disrupt recruiting, financing and supply chains quietly, all the way down to simply having a "friendly" chat with someone (and/or their mother).
The extend to which this happens and to what degree surveillance helps with this is difficult to ascertain, of course.
The extend to which this happens and to what degree surveillance helps with this is difficult to ascertain, of course.