The policy shift was intended to help the agency "discover and track" connections between intelligence targets overseas and people in the United States, according to an N.S.A. memorandum from January 2011. The agency was authorized to conduct "large-scale graph analysis on very large sets of communications metadata without having to check foreignness" of every e-mail address, phone number or other identifier, the document said. Because of concerns about infringing on the privacy of American citizens, the computer analysis of such data had previously been permitted only for foreigners.

The agency can augment the communications data with material from public, commercial and other sources, including bank codes, insurance information, Facebook profiles, passenger manifests, voter registration rolls and GPS location information, as well as property records and unspecified tax data, according to the documents. They do not indicate any restrictions on the use of such "enrichment" data, and several former senior Obama administration officials said the agency drew on it for both Americans and foreigners.

N.S.A. officials declined to say how many Americans have been caught up in the effort, including people involved in no wrongdoing. The documents do not describe what has resulted from the scrutiny, which links phone numbers and e-mails in a "contact chain" tied directly or indirectly to a person or organization overseas that is of foreign intelligence interest.