The case for expanded surveillance of communications, however, is complicated by an analysis of recent terrorist attacks. The Intercept has reviewed 10 high-profile jihadi attacks carried out in Western countries between 2013 and 2015 (see below), and in each case some or all of the perpetrators were already known to the authorities before they executed their plot. In other words, most of the terrorists involved were not ghost operatives who sprang from nowhere to commit their crimes.

Source : From Paris to Boston, Terrorists Were Already Known to Authorities