Each year, millions of visitors, foreign students, and immigrants come to the United States. A visitor may enter on a legal temporary basis—that is, with an authorized period of admission that expires on a specific date— either with a temporary visa (generally for tourism, business, or work) that the Department of State issues or, in some cases, as a tourist or business visitor who is allowed to enter without a visa. The latter category includes Canadians and qualified visitors from 27 countries who enter under the Visa Waiver Permanent program. A large majority of these visitors depart on time, but others overstay. The term “overstay” is defined as follows: An overstay is an illegal alien who was legally admitted to the United States for a specific authorized period but remained here after that period expired, without obtaining an extension or a change of status or meeting other specific conditions. Overstays who settle here are part of the illegal immigrant population. Although overstays are sometimes referred to as visa overstays, we do not use that term in this report for two reasons. First, many visitors are allowed to enter the United States without visas and to remain for specific periods of time, which they may overstay. Second, a visitor can overstay an authorized period of admission set by a U.S. Department of Homeland Security (DHS) inspector at the border—even though that authorized period may be shorter than the period of the visitor’s visa. (For example, a visitor with a 6-month multiple-entry visa from the Department of State might be issued a 6-week period of admission by the DHS inspector and remain here for 7 weeks, thus overstaying.) Viewed in terms of individuals, the overstay process can be summarized as aliens’ (1) legally visiting the United States, which for citizens of most nations is preceded by obtaining a passport and a visa and filling out Form I-94 at the U.S. border; (2) overstaying for a period that may range from a single day to weeks, months, or years; and, in some cases, (3) terminating their overstay status by exiting the United States or adjusting to legal permanent resident status (that is, obtaining a green card). Most long- term overstays appear to have economic motivations. However, the overstay process can also be viewed in the context of a layered defense for domestic security, supported by agencies such as DHS, the U.S. Department of Justice (DOJ), and the Department of State, among others. Figure 1 illustrates the layered-defense concept and the many interrelated issues that we have analyzed in numerous reports—ranging from the overseas tracking of terrorists to stateside security for critical infrastructure locations. Intelligence, investigation, and information sharing are the key ingredients supporting such a defense. A variety of immigration issues are potentially relevant. Progress in deploying and effectively using watch lists is ongoing. In 2003, we reported that (1) the State Department, “with the help of other agencies, almost doubled the number of names and the amount of information” in its Consular Lookout and Support System but that (2) “the federal watch list environment has been characterized by a proliferation of systems, among which information sharing is occurring in some cases but not in others.” Visitor biographical and biometric data are now being checked against selected watch list data, to verify visitors’ identity, as part of the new U.S. Visitor and Immigrant Status Indicator Technology program (US-VISIT). Keeping all dangerous persons and potential terrorist suspects from legally entering the United States is difficult because some do not match the expected characteristics of terrorists or suspicious persons. In addition, some—such as citizens of Canada or one of the 27 visa waiver countries— are not required to apply for visas and are not screened by the visa process. Terrorists may continue to slip through border defenses, and watch lists have therefore also been used for tracking foreign terrorists within the United States. Overstay tracking—that is, recording visitors’ entries and exits as well as their address information—also logically plays a role. Overstay issues have gained heightened attention because some of the hijackers of September 11, 2001, had overstayed their periods of admission. Form I-94 (shown in appendix I) is the basis of DHS’s long-standing system for tracking overstays. For visitors from most countries, the period of admission is authorized (or set) by a DHS inspector when they enter the United States legally and fill out this form. Each visitor is to give the top half of the form to the inspector and to retain the bottom half, which should be collected when the visitor departs the country. However, two major groups are exempt from filling out Form I-94 when they visit the United States for business or pleasure: Canadian citizens admitted for up to 6 months and Mexican citizens entering the United States with a border crossing card (BCC, illustrated in fig. 2) at the southwestern border who intend to limit their stay to less than 72 hours and intend not to travel beyond a set perimeter, generally 25 miles from the border (see app. II, fig. 6). During fiscal years 1999 to 2003, the Department of State issued 6.4 million Mexican BCCs. Because the majority of Canadian and Mexican BCC visits do not require Form I-94, the system based on this form cannot follow them—that is, cannot track them. No data indicate how many overstay. Overstay tracking should be possible for almost all other legal temporary visitors, including visitors from visa waiver countries, because they are required to fill out the form. describe available data on the extent to which overstaying occurs, identify any weaknesses that might limit the utility of DHS’s long-standing overstay tracking system, and provide some observations about the potential effect of overstays—as well as limitations of the overstay tracking system—on domestic security. In examining these issues, our main information sources included (1) relevant GAO and other reports, (2) interviews we conducted with officials and staff at DHS and DOJ, and (3) a variety of data, including printouts from DHS’s long-standing overstay tracking system (based on Form I-94), data that DHS developed, at our request, from Operation Tarmac (the sweep that identified overstays and other illegal immigrants working at U.S. airports) and other similar operations, and facts about the arrivals, departures, and overstay status of the September 11 hijackers and others involved in terrorist-related activities. We assessed the reliability of these data sources by reviewing existing information about the data, interviewing agency officials knowledgeable about the data and the process by which they were collected, and reviewing the data for reasonableness and corroboration with other independent data sources. While we found and reported on weaknesses in the data, we determined that the data were sufficiently reliable for the purposes of this report. Our scope did not include (1) aspects of illegal immigration or domestic security unrelated to overstaying or (2) elements of overstay enforcement additional to a system for tracking legal visitors’ entries and exits (for example, resource allocation). Our work was conducted in accordance with generally accepted government auditing standards between January 2003 and May 2004, primarily at DHS and DOJ headquarters in Washington, D.C. One visit to the southwest border was made to observe departure procedures. Significant numbers of visitors overstay their authorized periods of admission. A January 2003 DHS estimate put the January 2000 resident overstay population at one-third of 7 million illegal immigrants, or 2.3 million. While the method DHS used to obtain this figure is complex, indirect, and marked by potential weaknesses, we identified three small-sample alternative data points that, taken together, provide some evidence that, in all likelihood, a substantial proportion of illegal immigrants are overstays. These three alternative data sources on illegal immigrants indicate varying—but uniformly substantial—percentages of overstays: 31 percent, 27 percent, and 57 percent. At the same time, we found that DHS’s estimate excludes some overstay groups and may thus understate the extent of the total overstay problem. The main overstay groups omitted from the DHS overstay estimate of 2.3 million are long-term Mexican and Canadian overstays who were not required to fill out Form I-94 at entry and short-term overstays, whether from Mexico, Canada, or other countries. Short-term overstays cannot be ignored because, as we explain in a later section, some terrorists or terrorist supporters are in this group. DHS’s overstay estimate for January 2000 (that is, that overstays represent one-third of the illegal immigrant population, or 2.3 million residents) was based, in part, on a projection forward of overstay rates for 1992. Earlier, we identified challenges and potential weaknesses in Immigration and Naturalization Service (INS) procedures used in estimating these overstays (including an incorrect INS formula). Therefore, we sought alternative, and more current, data sources. The first alternative data source we identified is a survey that DHS and the National Institute of Child Health and Human Development sponsored, in partnership with other federal agencies. As reported in 2002, the survey (1) sampled more than 1,000 adult green-card holders, (2) asked them about their prior immigration status, and (3) found that more than 300 self- reported earlier illegal status. The computer run we requested showed that 31 percent of these former illegals said they had been overstays. (Most others reported prior illegal border crossing.) A second alternative source was a set of data we obtained from Operation Tarmac and other recent sweeps of employees who, in the course of their work, had access to sensitive areas in airports, other critical infrastructures, or special events (for example, the Super Bowl).