Congress has heard that surveillance programmes leaked by Edward Snowden helped to thwart 50 attacks since 2001.
The House intelligence committee was told by Army General Keith Alexander, director of the NSA, that the operations were vital.
The panel heard that efforts to launch an attack on the New York Stock Exchange were halted.
Gen Alexander, making his second public appearance before Congress since the surveillance programmes were exposed, said: "In recent years these programmes, together with other intelligence, have protected the US and our allies from terrorist threats across the globe to include helping prevent potential terrorist events over 50 times since 9/11."
Return to internet news headlines
View Internet News Archive