The National Security Agency has been secretly collecting the phone call records of tens of millions of Americans, using data provided by AT&T, Verizon and BellSouth, people with direct knowledge of the arrangement.

The NSA program reaches into homes and businesses across the nation by amassing information about the calls of ordinary Americans - most of whom aren't suspected of any crime. This program does not involve the NSA listening to or recording conversations. But the spy agency is using the data to analyze calling patterns in an effort to detect terrorist activity, sources said in separate interviews.

"It's the largest database ever assembled in the world," said one person, who, like the others who agreed to talk about the NSA's activities, declined to be identified by name or affiliation. The agency's goal is "to create a database of every call ever made" within the nation's borders, this person added.

Sources, however, say that is not the case. With access to records of billions of domestic calls, the NSA has gained a secret window into the communications habits of millions of Americans. Customers' names, street addresses and other personal information are not being handed over as part of NSA's domestic program, the sources said. But the phone numbers the NSA collects can easily be cross-checked with other databases to obtain that information.