WASHINGTON—The National Security Agency's monitoring of Americans includes customer records from the three major phone networks as well as emails and Web searches, and the agency also has cataloged credit-card transactions, said people familiar with the agency's activities.

The disclosure this week of an order by a secret U.S. court for Verizon Communications Inc. 's phone records set off the latest public discussion of the program. But people familiar with the NSA's operations said the initiative also encompasses phone-call data from AT&T Inc. and Sprint Nextel Corp., records from Internet-service providers and purchase information from credit-card providers.

The agency is using its secret access to the communications of millions of Americans to target possible terrorists, said people familiar with the effort.

The NSA's efforts have become institutionalized—yet not so well known to the public—under laws passed in the wake of the Sept. 11, 2001, attacks. Most members of Congress defended them Thursday as a way to root out terrorism, but civil-liberties groups decried the program.

"Everyone should just calm down and understand this isn't anything that is brand new,'' said Senate Majority Leader Harry Reid (D., Nev.), who added that the phone-data program has "worked to prevent'' terrorist attacks.