Officials in the government and the telecommunications industry who have knowledge of parts of the program say the N.S.A. has sought to analyze communications patterns to glean clues from details like who is calling whom, how long a phone call lasts and what time of day it is made, and the origins and destinations of phone calls and e-mail messages. Calls to and from Afghanistan, for instance, are known to have been of particular interest to the N.S.A. since the Sept. 11 attacks, the officials said.

This so-called "pattern analysis" on calls within the United States would, in many circumstances, require a court warrant if the government wanted to trace who calls whom.

The use of similar data-mining operations by the Bush administration in other contexts has raised strong objections, most notably in connection with the Total Information Awareness system, developed by the Pentagon for tracking terror suspects, and the Department of Homeland Security's Capps program for screening airline passengers. Both programs were ultimately scrapped after public outcries over possible threats to privacy and civil liberties.

But the Bush administration regards the N.S.A.'s ability to trace and analyze large volumes of data as critical to its expanded mission to detect terrorist plots before they can be carried out, officials familiar with the program say. Administration officials maintain that the system set up by Congress in 1978 under the Foreign Intelligence Surveillance Act does not give them the speed and flexibility to respond fully to terrorist threats at home.

A former technology manager at a major telecommunications company said that since the Sept. 11 attacks, the leading companies in the industry have been storing information on calling patterns and giving it to the federal government to aid in tracking possible terrorists.

"All that data is mined with the cooperation of the government and shared with them, and since 9/11, there's been much more active involvement in that area," said the former manager, a telecommunications expert who did not want his name or that of his former company used because of concern about revealing trade secrets.

Such information often proves just as valuable to the government as eavesdropping on the calls themselves, the former manager said.