But interior enforcement is also the most politically difficult part of immigration control. While there is at least nominal agreement on the need for improvements to the mechanics of visas and border monitoring, there is no elite consensus regarding interior enforcement. This is especially dangerous given that interior enforcement is the last fallback for immigration control, the final link in a chain of redundancy that starts with the visa application overseas.

There are two elements to interior enforcement: first, conventional measures such as arrest, detention and deportation; and second, verification of legal status when conducting important activities. The latter element is important because its goal is to disrupt the lives of illegal aliens so that many will return home on their own (and, in a security context, to disrupt the planning and execution of terrorist attacks).

Inadequacies in the first element of interior enforcement have clearly helped terrorists in the past. Because there is no way of determining which visitors have overstayed their visas, much less a mechanism for apprehending them, this has been a common means of remaining in the United States--of the twelve (out of 48) Al-Qaeda operatives who were illegal aliens when they took part in terrorism, seven were visa overstayers.

Among terrorists who were actually detained for one reason or another, several were released to go about their business inside America because of inadequate detention space. This lack of space means that most aliens in deportation proceedings are not detained, so that when ordered deported, they receive what is commonly known as a "run letter" instructing them to appear for deportation--and 94 percent of aliens from terrorist-sponsoring states disappear instead.

Lack of coordination between state and local police and federal immigration authorities is another major shortcoming. In the normal course of their work, police frequently encounter aliens. For instance, Mohammed Atta was ticketed in Broward County, Florida, in the spring of 2001 for driving without a license. But the officer had no mechanism to inform him that Atta had overstayed his visa during his prior trip to the United States. Although not an overstayer, another hijacker, Ziad Samir Jarrah, was issued a speeding ticket in Maryland just two days before 9/11, proving that even the most effective terrorists have run afoul of the law before launching their attacks.

Perhaps the most outrageous phenomenon in this area of conventional immigration enforcement is the adoption of "sanctuary" policies by cities across the country. Such policies prohibit city employees--including police--from reporting immigration violations to federal authorities or even inquiring as to a suspect's immigration status. It is unknown whether any terrorists have yet eluded detention with the help of such policies, but there is no doubt that many ordinary murderers, drug dealers, gang members and other undesirables have and will continue to do so.

The second element of interior enforcement has been, if anything, even more neglected. The creation of "virtual chokepoints", where an alien's legal status would be verified, is an important tool of immigration control, making it difficult for illegals to engage in the activities necessary for modern life.

The most important chokepoint is employment. Unfortunately, enforcement of the prohibition against hiring illegal aliens, passed in 1986, has all but stopped. This might seem to be of little importance to security, but in fact holding a job can be important to terrorists for a number of reasons. By giving them a means of support, it helps them blend into society. Neighbors might well become suspicious of young men who do not work but seem able to pay their bills. Moreover, supporting themselves by working would enable terrorists to avoid the scrutiny that might attend the transfer of money from abroad. Of course, terrorists who do not work can still arrive with large sums of cash, but this too creates risks of detection.

That said, the ban on employment by illegal aliens is one of the most widely violated immigration laws by terrorists. Among those who worked illegally at some point were CIA shooter Mir Aimal Kansi; Millennium plot conspirator Abdelghani Meskini; and 1993 World Trade Center bombers Eyad Ismoil, Mohammed Salameh and Mahmud and Mohammed Abouhalima.

Other chokepoints include obtaining a driver's license and opening a bank account, two things that most of the 9/11 hijackers had done. It is distressing to note that, while Virginia, Florida and New Jersey tightened their driver's license rules after learning that the hijackers had used licenses from those states, other states have not. Indeed, California's then-Governor Gray Davis signed a bill last year intended specifically to provide licenses to illegal aliens (which was repealed after his recall).

As for bank accounts, the trend is toward making it easier for illegal aliens to open them. The governments of Mexico and several other countries have joined with several major banks to promote the use of consular identification cards (for illegals who can't get other id) as a valid form of identification, something the U.S. Department of the Treasury explicitly sanctioned in an October 2002 report.

Finally, the provision of immigration services is an important chokepoint, one that provides the federal government additional opportunities to screen the same alien. There is a hierarchy of statuses a foreign-born person might possess, from illegal alien to short-term visitor, long-term visitor, permanent resident (green card holder) and finally, naturalized citizen. It is very beneficial for terrorists to move up in this hierarchy because it affords them additional opportunities to harm us. To take only one example: Mahmud Abouhalima--one of the leaders of the first World Trade Center bombing--was an illegal-alien visa overstayer; but he became a legal resident as part of the 1986 illegal-alien amnesty by falsely claiming to be a farmworker, and he was only then able to travel to Afghanistan for terrorist training and return to the United States.

There have been some improvements since 9/11 in the third layer of security. Illegal aliens who absconded after receiving their "run letters" are being, very slowly, added to the FBI's National Crime Information Center (NCIC) database, widely used by state and local police. What's more, the foreign-student tracking system is finally operational and the pilot program to develop a system for employers to verify the legal status of new hires was recently re-authorized and expanded.

But the ambivalence about interior enforcement is even deeper and more pronounced than in the two other layers of immigration control. There is a general sense among many political leaders that enforcing the immigration law is futile and, in any case, would displease important constituencies.

Former ins Commissioner James Ziglar expressed the general resistance to linking immigration law with homeland security when he said a month after the 9/11 attacks that "We're not talking about immigration, we're talking about evil." It is as if the terrorists were summoned from a magic lamp, rather than moving through our extensive but neglected immigration control system, by applying for visas, being admitted by inspectors, and violating laws with impunity inside America.

Upholding the Law

Such ambivalence about immigration enforcement, at whatever stage in the process, compromises our security. It is important to understand that the security function of immigration control is not merely opportunistic, like prosecuting Al Capone for tax violations for want of evidence on his other numerous crimes. The FBI's use of immigration charges to detain hundreds of Middle Easterners in the immediate aftermath of 9/11 was undoubtedly necessary, but it cannot be a model for the role of immigration law in homeland security. If our immigration system is so lax that it can be penetrated by a Mexican busboy, it can sure be penetrated by an Al-Qaeda terrorist.

Since there is no way to let in "good" illegal aliens but keep out "bad" ones, countering the asymmetric threats to our people and territory requires sustained, across-the-board immigration law enforcement. Anything less exposes us to grave dangers. Whatever the arguments for the president's amnesty and guest worker plan, no such proposal can plausibly be entertained until we have a robust, functioning immigration-control system. And we are nowhere close to that day.