WASHINGTON, D.C. -- Five years ago, Congress killed an experimental Pentagon antiterrorism program meant to vacuum up electronic data about people in the U.S. to search for suspicious patterns. Opponents called it too broad an intrusion on Americans' privacy, even after the Sept. 11 terrorist attacks.

But the data-sifting effort didn't disappear. The National Security Agency, once confined to foreign surveillance, has been building essentially the same system.

The central role the NSA has come to occupy in domestic intelligence gathering has never been publicly disclosed. But an inquiry reveals that its efforts have evolved to reach more broadly into data about people's communications, travel and finances in the U.S. than the domestic surveillance programs brought to light since the 2001 terrorist attacks.

Congress now is hotly debating domestic spying powers under the main law governing U.S. surveillance aimed at foreign threats. An expansion of those powers expired last month and awaits renewal, which could be voted on in the House of Representatives this week. The biggest point of contention over the law, the Foreign Intelligence Surveillance Act, is whether telecommunications and other companies should be made immune from liability for assisting government surveillance.

Largely missing from the public discussion is the role of the highly secretive NSA in analyzing that data, collected through little-known arrangements that can blur the lines between domestic and foreign intelligence gathering. Supporters say the NSA is serving as a key bulwark against foreign terrorists and that it would be reckless to constrain the agency's mission. The NSA says it is scrupulously following all applicable laws and that it keeps Congress fully informed of its activities.