Evidence showing that most of the terrorists involved in the September 11 attacks in New York and Washington entered the United States legally on student or tourist visas is prompting a re-examination of U.S. immigration and visa policies.

There are foreign students, tourists, business travelers and those with special work permits. In total, immigration lawyer Carl Shusterman estimates, about 100 million foreigners enter the United States annually. "The number of those who linger on beyond their planned stay, he says, is anybody's guess. "Supposedly the date they come into the U.S. and the date they're supposed to leave is entered into a computer system," he said. "But I really don't think the immigration service has the capability and the number of people they need to see if they left on time."

Mr. Shusterman says terrorist acts always create pressure for changing the system. He points to the 1979, when Iranians took over the U.S. Embassy in Tehran, as an example. "The same questions were brought up then because there were all these Iranian students in the United States who were not going to school any more," he said. "They put all sorts of systems in place to try to track students. You can see they really didn't work, because the number of students 20 years later dwarfs what we had in the late 70s."

Louise Krumm directs an English language program for foreign students at Georgetown University. Aside from personally placing every graduating student on a plane home, she says, there is no way of assuring those on student visas leave the country on time.

She is concerned about recent calls for a tracking system aimed at students, saying such a program might discourage foreigners from studying in the United States. "Last year there were over half a million students that came to study, and they bring $12.3 billion into the United States, and this is a very negative message to them," she said.

In the same way, Carl Shusterman says, a national system that tracks every foreigner would greatly curb tourism. "If we really tracked everybody, we would be a lot more like the Soviet Union," said Carl Shusterman. "If a tourist group came over they could not go out for a bite to eat unless they checked with the Intourist guide. I guess it could be done, but we would be living in a far different country if these measures went into place."

There is safety and then there is freedom, Carl Shusterman says. Despite the concerns prompted by the recent terrorist attacks, he says, most Americans are unwilling to sacrifice freedom for safety.