In a rare statement on normally top secret matters the director of the NSA, General Keith Alexander, said the operations had “helped prevent more than 50” attacks around the world, according to The Guardian.

Alexander, testifying to US Congress, described the PRISM surveillance of telephone and internet communications around the world as “limited, focused and subject to rigorous oversight”.

“I would much rather today be here to debate this point than try to explain why we failed to prevent another 9/11,” said the general, who detailed some examples of instances when terrorist activity was intercepted.

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