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Border Security: Terrorists

One of the primary methods for preventing foreign terrorist attacks on U.S. soil is to prevent individuals and groups from entering the country in the first place. Recent data released by the U.S. Department of Homeland Security (DHS) shows that some individuals from the Terrorist Screening Dataset (TSDS) have tried to enter the U.S. at ports of entry at the border with Mexico. Others have been detained after attempting to enter the U.S. illegally between ports of entry. Do these cases point to an increased risk of future terrorist attacks on U.S. citizens?

What is the Terrorist Screening Dataset?

The Terrorist Screening Dataset (TSDS) is maintained by the Federal Bureau of Investigation’s National Counterterrorism Center. It contains the names of over a million individuals identified by U.S. intelligence agencies as known or suspected terrorists. It was developed after the 9/11 attacks to make it easier for intelligence agencies to share this information with federal, state, and local law enforcement agencies as a way of preventing future terrorist attacks. An individual is added to the TSDS only if there is a “reasonable suspicion” that they are a known or suspected terrorist. 

One important point is that the TSDS only identifies individuals as known or suspected terrorists. It does not include information on whether they threaten the U.S.. For example, the TSDS includes the names of individuals who are members of FARC, a terrorist organization operating against the Colombian government. The FARC has never organized an attack in the U.S. and, in fact, no longer conducts terrorist operations in Columbia. 

Another problem with using the TSDS to flag suspected terrorists is the possibility of a false positive. In this situation, an individual’s name matches an entry on the TSDS but is a different person with the same name. There have been cases where someone has been arrested because of a false positive and later released when the discrepancy was realized. 

What evidence is there for terrorists entering the U.S. from Mexico?

The Department of Homeland Security data for 2017 – 2023 is shown below.

Source: Customs and Border Patrol (2023)

Encounters at ports of entry are cases where someone entering the U.S. at a Border Patrol facility is detained because a passport check shows they are on the TSDS. Encounters between ports of entry are cases where someone tries to enter the country illegally but is apprehended by law enforcement and later identified as being on the TSDS.

As the figure shows, both kinds of encounters are on the rise. As we discuss further in our Policy Brief on Border Security, there has been a sharp increase in border crossings in recent years. For more details, see the brief linked in the Further Reading section.

The increase in TSDS encounters between ports of entry is particularly concerning, as these individuals are trying to enter the U.S. without having their identity checked. The encounter data is reassuring at one level, showing that many terror suspects are being identified and detained. However, DHS data for 2023 also shows hundreds of thousands of “getaways” – encounters between ports of entry where an individual evades arrest. While many of these getaways are undoubtedly ordinary people who are escaping political persecution or economic hardship in their home countries, the concern is that potential terrorists are using this entry method to evade detection. 

How high is the risk?

Does the sharp increase in TSDS encounters outside ports of entry imply an increased risk of a domestic terrorist attack? It is impossible to be sure, however, a concern is that as the number of illegal entrants rises, it might be easier for a would-be terrorist to enter the country undetected.

A detailed study of border security and terrorist attacks over the last 30 years confirms the need for vigilance but also suggests that analyses based on TSDS data need to be placed in context:

  • There do not appear to be any cases of a person on the TSDS who entered the country illegally and was later indicted for planning or carrying out a terrorist attack.
  • No Americans have been killed by terrorists who entered the country illegally. (Of the 19 individuals who participated in the 9/11 attacks, all entered the U.S. legally using passports and visas.)
  • There are cases where the TSDS has flagged individuals not planning attacks against the U.S.. For example, 25 of the 27 people flagged by Border Patrol TSDS checks in 2022 were Colombian citizens and likely members of the FARC. Why they were trying to enter the U.S. is unknown, but it is unlikely that they were planning a terrorist attack.


The increase in the number of Border Patrol encounters with individuals on the TSDS is part of a larger problem with maintaining border security given the large number of individuals seeking to enter the U.S.. However, the issue of would-be terrorists entering the country is a different problem, one that requires scrutiny of all entrants, not just asylum applicants. 

The TSDS data also raise a fundamental question about border security: how much is enough? There is no doubt that the number of illegal border crossings by individuals on the TSDS could be reduced through increased surveillance of the border in between ports of entry or even by deploying military assets to secure the border. The fact remains, however, U.S. records indicate that no illegal entrant has ever been directly linked to a terrorist attack on U.S. soil. This suggests that while greater border security would reduce some of the underlying challenges with immigration, these measures may still have little impact on preventing a terrorist attack in the future.


Further Reading

Policy vs Politics Policy Brief: Border Security.

Federal Bureau of Investigation. (2023). Terrorist Screening Center: Frequently Asked Questions., accessed 2/12/24.

Mueller, J. & M. Stewart. (2011). Terror, Security, and Money: Balancing the Risks, Benefits, and Costs of Homeland Security. Oxford University Press.



What is the Terrorist Screening Dataset?

Federal Bureau of Investigation. (2023). Terrorist Screening Center: Frequently Asked Questions., accessed 2/12/24.

American Civil Liberties Union. (2018). Overview of the U.S. Government’s Watchlisting Process and Procedures., accessed 2/12/24.

What evidence is there for terrorists entering the U.S. from Mexico?    

Customs and Border Patrol. (2024). CBP Enforcement Statistics., accessed 2/12/24.

Felbab-Broen, V. (2021). 9-11 and the U.S.-Mexico border: New challenges 20 years later. Brookings Institution., accessed 2/12/24

Congressional Research Service. (2019). Border Security Between Ports of Entry., accessed 2/12/24.

How high is the risk?

Mueller, J. & M. Stewart. (2011). Terror, Security, and Money: Balancing the Risks, Benefits, and Costs of Homeland Security. Oxford University Press.

Subcommittee on Immigration Integrity, Security, and Enforcement, Committee on the Judiciary. (2023). Terrorist Entry Through the Southwest Border, testimony before the. United States House of Representatives., accessed 2/12/24.



Julia Acevedo (Intern) is a Political Science and Public Policy double major at Susquehanna University and is expected to graduate in May 2024 and pursue a Masters degree in Public Health.

Elijah Oaks (Intern) is a student at Dartmouth College. He is expected to graduate in May 2024 with a major in English and a minor in Religion. He is a Policy Fellow at The Cicero Institute.

Dr. William Bianco (Research Director) received his PhD in Political Science from the University of Rochester. He is Professor of Political Science and Director of the Indiana Political Analytics Workshop at Indiana University. His current research is on representation, political identities, and the politics of scientific research.


Publication Log

Published 4/2/24

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