A friend asked me, "Why doesn't the NSA, for all their data harvesting, actually seem to prevent terror attacks?"

Basically, the latency of the justice system makes that impossible, but they will be able to predict the future.

At the moment, many of the NSA's projects and systems are very new: it's only recently they've been able to bring together the technologies that have developed in the last decade, such as neural net, and those technologies are not yet trained to understand the NSA's raw data.

Now that we're finally able to take the theory of algorithmic data-to-intelligence flow out of the classroom, we have to figure out how to apply it to the real world.

Think of it like physics. It's pretty easy to figure out gravity, because there's a lot of material to observe. But if there were only 8 billion particles to be observed, what is self-evident as gravity in a universe of aummillions of particles, would just look like random static (unless observed for an unreasonable amount of time, for the analogy to hold). Because the NSA is only dealing with the data of around 600 million individuals, and reasonable useful amount of data on only say 250 million, it is very hard to infer any sort of meaningful knowledge from that data.

With an increased period and quantity of information, however, combined with development of proper maths for dealing with the data, the NSA will eventually be able to make sense of all the information it's capturing today.

Also, using the sort of data which can be scrapped by the NSA and manually interpreted, it is still very difficult to ascertain the specific information about any given action, as obtaining that data requires the more explicit involvement of our justice system. What reveals itself more are general trends in social groups and individual dynamics. It becomes easy to see that a neighborhood's rate of crime is going to increase, or that a certain criminal operation is going to become violent, sure, but knowing exactly how will be a human-operated procedure for a long time to come.

This gives the NSA a potentially unique outlook on our nation, as they will be able to, essentially, predict the future. Not the specifics of it, but with increasing accuracy, the forces which act our our social layout the way gravity works, unseen, on our universe of 8 billion particles.

One possible implication of this is a factor in the plot of Isaac Asimov's Foundation series, which I'll close by recommending.