Without their knowledge, millions of Americans and foreigners crossing U.S. borders in the past four years have been assigned scores generated by U.S. government computers rating the risk that the travelers are terrorists or criminals.

The travelers are not allowed to see or directly challenge these risk assessments, which the government intends to keep on file for 40 years.

The government calls the system critical to national security following the Sept. 11, 2001, terrorist attacks. Some privacy advocates call it one of the most intrusive and risky schemes yet mounted in the name of anti-terrorism efforts.

Virtually every person entering and leaving the United States by air, sea or land is scored by the Homeland Security Department's Automated Targeting System, or ATS. The scores are based on ATS' analysis of their travel records and other data, including items such as where they are from, how they paid for tickets, their motor vehicle records, past one-way travel, seating preference and what kind of meal they ordered.

The use of the program on travelers was quietly disclosed earlier this month when the department put a notice detailing ATS in the Federal Register, a fine-print compendium of federal rules. The few civil liberties lawyers who had heard of ATS and even some law enforcement officers said they had thought it was only used to screen cargo.

The Homeland Security Department called the program "one of the most advanced targeting systems in the world" and said the nation's ability to spot criminals and other security threats "would be critically impaired without access to this data."

But to David Sobel, a lawyer at the Electronic Frontier Foundation, a group devoted to civil liberties in cyberspace: "It's probably the most invasive system the government has yet deployed in terms of the number of people affected."

Government officials could not say whether ATS has apprehended any terrorists. Based on all the information available to them, federal agents turn back about 45 foreign criminals a day at U.S. borders, according to Homeland Security's Customs and Border Protection spokesman Bill Anthony. He could not say how many were spotted by ATS.

"Homeland Security ought to focus on the simple things it can do and stop trying to build these overly complex jury-rigged systems," said Barry Steinhardt, an American Civil Liberties Union lawyer, citing problems the agency has had developing a computerized screening system for domestic air travelers.