I'm a little skeptical of the conclusions drawn from these questions, especially when they're phrased in terms of surveillance data directly preventing specific

I'm a little skeptical of the conclusions drawn from these questions, especially when they're phrased in terms of surveillance data directly preventing specific attacks. From experience elsewhere it seems (I claim no expertise here) that you rarely begin looking a terrorist's or potential terrorist's communications at just the right time (neither too early nor too late) to discover a planned terrorist attack for the first time. What happens more often, apparently, is that you start looking at someone's communications and you gather information useful to turning them or someone else into an informer. Then you discover attack plans drawn up in the future because your double agent calls you up and tells you about them directly. If the US is in fact using domestic surveillance effectively to generate informers, and thus prevent terror attacks, then US alphabet-soup spokesmen might plausibly be very slow to mention much about it in response to questions asked about specific attacks being directly detected through surveillance, both because a) the question as phrased doesn't cover such operations and b) you want at almost any cost to say nothing about them. Though on the other hand, if you aren't having success in those operations you wouldn't feel like saying much about it anyway, partly because it means you're failing at counter-terrorism ...