The Sept. 11 terrorists didn't have to steal into the country as stowaways on the high seas or as border jumpers dodging federal agents. No audacious enemy "inserted" them, commando style.

Most or all appear to have come in legally, on the kinds of temporary visas routinely granted each year to millions of foreign tourists, merchants, students and others.

Nothing in the backgrounds of these middle-class men from Saudi Arabia, Egypt and elsewhere apparently aroused suspicion among State Department consular officers who review visa applications. And, once here, the 19 hijackers-to-be didn't have to fret much about checkpoints and police stops, even after some of their visas expired and they became illegal immigrants.

The suicide attacks that killed 6,000 or more have brutally exposed shortcomings in airline security and intelligence gathering. But the strikes also highlighted another vulnerability: the nation's visa-granting and immigration regimen.

The entire system is principally geared toward meeting another kind of threat: people of modest means whose concealed aim is not to bomb and wreak havoc but to work illegally in the United States. Moreover, proposals by Congress to keep closer track of immigrants living in the United States have been delayed or blocked because of complaints that the new rules would be too restrictive.

"I worry about the whole screening process," said Rep. George W. Gekas, R-Pa., who heads the House subcommittee on immigration and claims, which will be considering broad changes in the aftermath of Sept. 11. "It has been rather porous in the past, and we have to strengthen it. What the incidents have done is raise the awareness of everybody."

However, any prospective reforms require a delicate balancing act. The United States records about 500 million legal entries each year. Officials must weigh the benefits of tighter enforcement against the value of free movement of people and goods.

"The kinds of enforcement and the kinds of controls you impose on the immigration system have to be done in a way that does not substantially impede legitimate travel and commerce, because it's our lifeblood," said former Immigration and Naturalization Service Commissioner Doris Meissner. "The vast majority of people coming here are law-abiding and good for the country. That's the balance that has to be struck."

What little is known of the hijackers' history in this country suggests a certain confidence that immigration law can be circumvented where necessary.

For example, confidential records indicate that two possible hijacking ringleaders -- Mohamed Atta and Marwan Al-Shehhi, presumed pilots of the jets that hit the World Trade Center -- overstayed their initial visas. It is an abuse that can void the travel document. Yet despite having no valid visas, both men left the country and were allowed to return on flights through Miami and New York last January, said an INS official who reviewed the records.

Other hijackers may have been in the country on lapsed or otherwise invalid visas, authorities say. Officials declined to provide more specifics.

Yet, all the conspirators could have confidently stayed below the immigration enforcement radar screen amid the multitudes of foreign nationals who go on with their lives despite the prospect of deportation.

A similar pattern emerged with other foreign terrorist strikes on U.S. soil.

A Palestinian convicted of driving a van packed with explosives into the World Trade Center in 1993 received a student visa. He dropped out of school, and the INS never found him. The alleged mastermind of that bombing, another Palestinian, had a pending claim of political asylum. That same year, a Pakistani given a temporary work permit because of his political asylum application went on a shooting rampage outside CIA headquarters in Virginia.

Outraged critics on Capitol Hill and elsewhere are calling for a clampdown on visas now liberally granted to students and visitors. Among the proposals being circulated are tightened visa screening, mandatory waiting periods for visa applicants, systematic tracking of all foreign nationals in the United States and even the creation of a national identity card -- a notion long assailed by privacy advocates on the right and left.

"Right now we have no ability to identify, locate or remove foreigners who deliberately remain in this country long after their tourist or student visas expire," said Sen. Christopher S. Bond, R-Mo. "That is intolerable."

As many as 4 million one-time legal tourists, students and others have become illegal immigrants, according to government and academic estimates.

Federal officials acknowledge they have no idea where all these people are.

Congress has long hesitated to fund immigration enforcement efforts in the nation's interior, in contrast to the lavish budget increases bestowed upon the Border Patrol. Raids of factories, fields and other sites where illegal immigrants find work have never been popular with business.