The nation's system for screening and tracking foreign visitors is antiquated, cumbersome and overwhelmed. As a result, the immigration system provides no reliable means of finding potential terrorists who have violated the terms of their visas.

Visa weaknesses The screening and tracking system suffers from: • A crush of would-be foreign visitors that has overwhelmed the U.S. State Department's consular corps, which screens applicants and grants visas. • Daily pressure at U.S. border checkpoints leaves little time to examine individual foreigners. • An inability to track foreign tourists, businesspeople or students who stay longer than their visas permit. • Lack of a departure system that accounts for foreigners as they leave the country, a failing that means authorities have no way of knowing which foreigners are still here and which have left. • A program that waives even basic visa requirements for millions of visitors.

Authorities estimate that, as a result of the holes in the current system, 40 percent of the foreigners living illegally in the United States were legally approved for temporary stays but never left. That means about 4 million foreigners in the United States have overstayed their visas, a number that grows by 125,000 each year.

That number can include terrorists. The 19 who hijacked four airplanes and used them Sept. 11 to attack New York and the Pentagon apparently entered on student, business and tourist visas, and at least three overstayed. Many other suspected terrorists or material witnesses now in federal custody are being held on immigration violations, including visa overstays.

Federal agencies and Congress have attempted to upgrade the system. But, until now, business, higher-education and civil-liberties groups have thwarted attempts to modernize it.

"You will see a new attitude toward entry and exits systems," says Rep. Lamar Smith, R-Texas, who has backed several tough anti-terrorist measures in Congress. "We clearly need to know who is coming in, why they're staying and whether they left the country."

The problem extends beyond U.S. borders, he adds. "The Canadian and the U.S. governments are going to have to collaborate on this," he says, "because Canada is a haven for terrorists, and the United States is, too."

Border authorities have more than 500 million encounters with people who want to enter the United States each year. Two-thirds of those are with noncitizens. Many are repeat encounters; all strain an overloaded system.

Each year about 9 million foreigners apply for visas to visit the United States. Consular officers grant 6 million of those. The 3 million rejected are turned down because, among other things, they are security risks, have diseases or fail to convince officers they ultimately will leave the United States.

Few of the consular officers charged with reviewing those millions of applications are high-level, experienced officials. Money cuts have left hundreds of consular-officer jobs unfilled.

"The consular corps is overwhelmed," says Bill Dameron, former U.S. ambassador to Mali.

"When I was there," adds Dameron, who retired to Seattle in 1995 after a 27-year foreign-service career, "they needed much better technology; they needed better everything."

The system is not completely broken. Since the 1993 World Trade Center bombing, the State Department has rejected hundreds of suspected and known terrorists using a watch list and visa committees that alert the department to suspicious foreigners. The Immigration and Naturalization Service (INS) and Customs Service also have intercepted potential terrorists.

But millions of foreigners entering the United States each year avoid even the investigation that accompanies most visa applications. Authorities waive the visa process for more than 17 million people, including citizens of Portugal, Uruguay, Singapore, Germany and Belgium.

The visa-waiver program began in 1986 as a temporary measure to whisk British and Japanese businesspeople through borders with just a passport if they intended to stay fewer than 90 days. The program proved so popular that it was renewed and expanded to 27 other countries.

In October, President Clinton made the program permanent. He did so despite the U.S. Inspector General's grave concerns over the potential for abuse, including a substantial jump in passport thefts in visa-waiver countries.

Laws passed in 1996 require the INS to better track who enters and departs the country and to monitor foreigners on student visas. The INS instituted or proposed programs for both missions, but political pressures gutted the efforts.

A pilot program to automatically track air-travel arrivals and exits has stalled. After four years it covers only 14 airports.