The NSA head has effectively admitted that the US gains no real security against terrorist attacks from massive general surveillance.

Posing the question in terms of whether massive surveillance has ever prevented any violence in the US is a gratuitous concession to the NSA.

The suppression of leaks is extremely dangerous, in the same way a nuclear power plant is dangerous: there's a small but substantial chance it will cause a massive catastrophe (fallout, for the nuclear plant; tyranny, for the War on Journalism). The September 2001 attack were a pinprick compared with this.

Even if massive surveillance prevented one attack on that scale each decade, that would not justify massive surveillance. 300 additional deaths per year, set against the population of the US, is a tiny risk. In the Land of the Free and the Home of the Brave, we can pay that price to maintain our freedom.