In the weeks following the terrorist attacks of September 11 , 2001 , the President of the United States authorized the National Security Agency ( NSA ) to conduct a classified program to detect and prevent further attacks in the United States . As part of the NSA 's classified program , several different intelligence activities were authorized in Presidential authorizations , and the details of these activities changed over time . The program was reauthorized by the President approximately every 45 days , with certain modifications . Collectively , the activities carried out under these authorizations are referred to as the " President 's Surveillance Program " ( PSP )
terrorist attacks of September 11 , 2001
September 11 attacks
President's Surveillance Program