It's long been one of the biggest holes in America's security net: foreigners who come to the US legally and then overstay their visas - often without the government knowing.
Four of the 9/11 hijackers had expired visas. The General Accounting Office estimates the number of visa overstays at 2 million - and growing by 125,000 a year.
Now a rigorous new tracking system is being put into effect, designed to be more foolproof and to enhance border security in an age of terrorism. By helping authorities track just which visitors are here and which have left, proponents hope, the system will give a better sense of who remains in the US.
The new system, which the Department of Homeland Security started implementing at 14 major seaports and 115 airports this week, uses digital photographs, fingerprint scans, and biometrics - the unique physical traits that verify an individual's identity. And for the first time, it's exposing certain people leaving the country to the same checks as those coming in.
Critics see the plan - the United States Visitor and Immigration Status Indicator Technology (or US VISIT) program - as intrusive and cumbersome, damaging trade and tourism, and threatening to bring ports of entry to a grinding halt.
But supporters see it as essential to keeping the United States safe at a time when the Al Qaeda threat is omnipresent - and the added delay, according to preliminary runs in a pilot program at Atlanta's Hartsfield-Jackson Airport this fall, runs only about 15 seconds per person.
"Visa overstays are not a trivial matter," says Mark Krikorian, executive director of the Center for Immigration Studies in Washington, which favors tougher immigration laws. "A third to a half of all those in the United States illegally came in legally and overstayed their visas."
The only exceptions to the new checks are visitors from 28 (mostly European) nations, whose citizens are permitted to visit the United States for 90 days without visas. In all, US VISIT will track an estimated 24 million foreigners each year.
Virtually everyone finds fault with the old visa-checking system, in which foreigners leaving the country were asked to hand a copy of their visas to flight attendants, who then passed it on to the airlines. The airline passed it on to the Immigration and Naturalization Service (INS), which contracted with a company to enter the information into a database.
Even the INS has admitted the old database was practically useless.
The new program is designed to streamline the process and enhance border security, with scanned fingerprints and photographs checked instantly against a national digital database for criminal backgrounds and terrorist lists.
The Immigration Reform Act of 1996 was supposed to introduce a similar tracking system, but its start date was postponed twice. After Sept. 11, 2001, Congress became emphatic about the need for imminent reform.
But pro-immigration and tourism groups have criticized the speed of implementation. The American Immigration Lawyers Association, for instance, says the program is "overly ambitious and is likely to result in a deeply flawed system."
The GAO agreed in a September report, calling the US VISIT program a risky endeavor with daunting goals, high costs, and details that had yet to be worked out. The report also warned of long lines at ports of entry.
Department of Homeland Security officials insist that the system, which will snap a traveler's photo while his fingerprints are being taken, will add only minutes to each schedule even in its first weeks - and only seconds as screeners become more proficient.
But some say those numbers are misleading. Judith Golub, senior director of advocacy at the American Immigration Lawyers Association, points to a 1998 Senate committee report that looked at the issue of time associated with a new entry/exit system. It concluded that adding 30 seconds to each person's crossing at the Peace Bridge in Buffalo, N.Y., would result in a backup of 2-1/2 days.
"Of course we should enhance our national security, but we should find a balance that allows for the flow of people and goods," says Ms. Golub. "If we don't do that, we won't have economic security. And if we don't have economic security, we won't be able to afford our national security."
Indeed, economic stakes are high. Mexican officials, for example, concluded that their residents spent $3.6 billion on the US side of the Southwest border in 2002.
That makes US border towns especially jittery about the new system, which is scheduled to be installed at the 50 busiest land crossings by the end of 2004. These land crossings handle about 80 percent of the 500 million entry inspections each year, and heightened security has already put some US border businesses in jeopardy.
But Mr. Krikorian, for one, has little sympathy when national security is in question. "The lives of my children are not dependent on the financial success of Wal-Mart," he says. "Making sure we have a working, secure immigration system should be the first function of the government ... and this is an important step towards that."
• Material from wire services was used in this report.