After almost every school shooting in America or terrorist attack in Europe it turns out that the perpetrator was known or monitored by police or security services. So, obtaining information wasn't the problem. Properly acting on it was; it required more resources than most governments in the world were willing or able to do.
I mean, it is nothing new. After JFK assassination FBI was caught destroying files about Lee Harvey Oswald, a dangerous radical who was, like so many at the time were (and so many right are now) under some sort of surveillance. When some of those radicals actually do something bad, it is much easier to destroy files than to explain why so much of taxpayers money or so many civil liberties were sacrificed for naught.
RE: Data Dump: Why Retrospect Datamining is Foolish and Sensational