he Pentagon is constructing a computer system that could create a vast electronic dragnet, searching for personal information as part of the hunt for terrorists around the globe — including the United States.

As the director of the effort, Vice Adm. John M. Poindexter, has described the system in Pentagon documents and in speeches, it will provide intelligence analysts and law enforcement officials with instant access to information from Internet mail and calling records to credit card and banking transactions and travel documents, without a search warrant.

Historically, military and intelligence agencies have not been permitted to spy on Americans without extraordinary legal authorization. But Admiral Poindexter, the former national security adviser in the Reagan administration, has argued that the government needs broad new powers to process, store and mine billions of minute details of electronic life in the United States.

Admiral Poindexter, who has described the plan in public documents and speeches but declined to be interviewed, has said that the government needs to "break down the stovepipes" that separate commercial and government databases, allowing teams of intelligence agency analysts to hunt for hidden patterns of activity with powerful computers.

"We must become much more efficient and more clever in the ways we find new sources of data, mine information from the new and old, generate information, make it available for analysis, convert it to knowledge, and create actionable options," he said in a speech in California earlier this year.

Admiral Poindexter quietly returned to the government in January to take charge of the Office of Information Awareness at the Defense Advanced Research Projects Agency, known as Darpa. The office is responsible for developing new surveillance technologies in the wake of the Sept. 11 attacks.

In order to deploy such a system, known as Total Information Awareness, new legislation would be needed, some of which has been proposed by the Bush administration in the Homeland Security Act that is now before Congress. That legislation would amend the Privacy Act of 1974, which was intended to limit what government agencies could do with private information.

The possibility that the system might be deployed domestically to let intelligence officials look into commercial transactions worries civil liberties proponents.

"This could be the perfect storm for civil liberties in America," said Marc Rotenberg, director of the Electronic Privacy Information Center in Washington "The vehicle is the Homeland Security Act, the technology is Darpa and the agency is the F.B.I. The outcome is a system of national surveillance of the American public."

Secretary of Defense Donald H. Rumsfeld has been briefed on the project by Admiral Poindexter and the two had a lunch to discuss it, according to a Pentagon spokesman.

"As part of our development process, we hope to coordinate with a variety of organizations, to include the law enforcement community," a Pentagon spokeswoman said.

An F.B.I. official, who spoke on the condition that he not be identified, said the bureau had had preliminary discussions with the Pentagon about the project but that no final decision had been made about what information the F.B.I. might add to the system.

A spokesman for the White House Office of Homeland Security, Gordon Johndroe, said officials in the office were not familiar with the computer project and he declined to discuss concerns raised by the project's critics without knowing more about it.

He referred all questions to the Defense Department, where officials said they could not address civil liberties concerns because they too were not familiar enough with the project.

Some members of a panel of computer scientists and policy experts who were asked by the Pentagon to review the privacy implications this summer said terrorists might find ways to avoid detection and that the system might be easily abused.

"A lot of my colleagues are uncomfortable about this and worry about the potential uses that this technology might be put, if not by this administration then by a future one," said Barbara Simon, a computer scientist who is past president of the Association of Computing Machinery. "Once you've got it in place you can't control it."

Other technology policy experts dispute that assessment and support Admiral Poindexter's position that linking of databases is necessary to track potential enemies operating inside the United States.

"They're conceptualizing the problem in the way we've suggested it needs to be understood," said Philip Zelikow, a historian who is executive director of the Markle Foundation task force on National Security in the Information Age. "They have a pretty good vision of the need to make the tradeoffs in favor of more sharing and openness."

On Wednesday morning, the panel reported its findings to Dr. Tony Tether, the director of the defense research agency, urging development of technologies to protect privacy as well as surveillance, according to several people who attended the meeting.

If deployed, civil libertarians argue, the computer system would rapidly bring a surveillance state. They assert that potential terrorists would soon learn how to avoid detection in any case.

The new system will rely on a set of computer-based pattern recognition techniques known as "data mining," a set of statistical techniques used by scientists as well as by marketers searching for potential customers.

The system would permit a team of intelligence analysts to gather and view information from databases, pursue links between individuals and groups, respond to automatic alerts, and share information efficiently, all from their individual computers.

The project calls for the development of a prototype based on test data that would be deployed at the Army Intelligence and Security Command at Fort Belvoir, Va. Officials would not say when the system would be put into operation.

The system is one of a number of projects now under way inside the government to lash together both commercial and government data to hunt for patterns of terrorist activities.

"What we are doing is developing technologies and a prototype system to revolutionize the ability of the United States to detect, classify and identify foreign terrorists, and decipher their plans, and thereby enable the U.S. to take timely action to successfully pre-empt and defeat terrorist acts," said Jan Walker, the spokeswoman for the defense research agency.

Before taking the position at the Pentagon, Admiral Poindexter, who was convicted in 1990 for his role in the Iran-contra affair, had worked as a contractor on one of the projects he now controls. Admiral Poindexter's conviction was reversed in 1991 by a federal appeals court because he had been granted immunity for his testimony before Congress about the case.