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; they were already viewed as a potential threat, yet were not subjected to sufficient scrutiny by authorities under existing counterterrorism powers. Some of those involved in last week’s Paris massacre, for instance, were already known to authorities; at least three of the men appear to have been flagged at different times as having been radicalized, but warning signs were ignored.

In the aftermath of a terrorist atrocity, government officials often seem to talk about surveillance as if it were some sort of panacea, a silver bullet. But what they always fail to explain is how, even with mass surveillance systems already in place in countries like France, the United States, and the United Kingdom, attacks still happen. In reality, it is only possible to watch some of the people some of the time, not all of the people all of the time. Even if you had every single person in the world under constant electronic surveillance, you would still need a human being to analyze the data and assess any threats in a timely fashion. And human resources are limited and fallible.