NSA director says surveillance helped stop 'dozens' of attacks

WASHINGTON (Reuters) - The head of the National Security Agency said on Wednesday that extensive U.S. surveillance efforts had helped stop "dozens" of possible attacks, and warned that making details of the top-secret programs public had compromised national security.

In his first appearance before Congress since an NSA contractor lifted the veil on the agency's broad monitoring of phone and internet data, General Keith Alexander defended the program as an essential tool in the fight against terrorism.