Legal Authorities Supporting the Activities of the National Security Agency Described by the President [open pdf - 293KB]
"As the President has explained, since shortly after the attacks of September 11, 2001, he has authorized the National Security Agency (NSA) to intercept international communications into and out of the United States of persons linked to al Qaeda or related terrorist organizations. The purpose of these intercepts is to establish an early warning system to detect and prevent another catastrophic terrorist attack on the United States. This paper addresses, in an unclassified form, the legal basis for the NSA activities described by the President ('NSA activities')."
Federation of American Scientists: http://www.fas.org