Early this year, the Department of Defense disclosed the most sweeping effort to monitor the activity of Americans since the 1960's, a program called Total Information Awareness. The T.I.A. program is the Bush administration's most visible attempt to implement an idea that became ascendent after 9/11: that the best way to catch terrorists is to allow federal agencies to share information about American citizens and aliens that is currently stored in separate databases. When Congress created the Department of Homeland Security, for instance, it pledged to share data with state and local officials that is currently maintained by the F.B.I. and the C.I.A.

But the Total Information Awareness program takes the principle of information-sharing to a new level. Directed by John Poindexter, the former national security adviser to President Reagan whose conviction for lying to Congress was overturned on appeal, the T.I.A. program seeks to ''revolutionize the ability of the United States to detect, classify and identify foreign terrorists'' by developing data-mining and profiling technologies that could analyze commercial transactions and private communications.

According to its Web site, which features a Latin slogan that means ''knowledge is power,'' ''Total Information Awareness of transnational threats requires keeping track of individuals and understanding how they fit into models.'' To this end, T.I.A. seeks to develop architectures for integrating existing databases into a ''virtual, centralized, grand database.'' In addition to analyzing financial, educational, travel and medical records, as well as criminal and other governmental records, the T.I.A. program could include the development of technologies to create risk profiles for millions of visitors and American citizens in its quest for suspicious patterns of behavior.

Civil libertarians greeted T.I.A. with alarm and called it a harbinger of even more Orwellian technologies to come. ''I fully believe that data-mining in the next five years will be used to determine whether you or I get access to a federal office building,'' said Marc Rotenberg of the Electronic Privacy Information Center. The Bush administration stands behind T.I.A., but privacy advocates hope to persuade Congress to pull the plug. When the government proposed creating a National Data Center in 1965, public outcry led to the passage of the Privacy Act of 1974, which prohibits federal agencies from routinely sharing personal information. Whether Americans still support that principle after 9/11 remains to be seen. Jeffrey Rosen