Are terrorists slipping across the US border? What the evidence shows.

In recent months, a string of national security figures has warned Congress there is a heightened threat of a terrorist attack on U.S. soil.

“Looking back over my career in law enforcement, I’d be hard-pressed to think of a time when so many different threats to our public safety and national security were so elevated all at once,” FBI Director Christopher Wray testified this spring. Part of that, he said, is an elevated threat of foreign terrorist organizations attacking the United States following the Hamas cross-border raid on Israel Oct. 7. “Obviously, their ability to exploit any point of entry, including our southwest border, is a source of concern,” he said in December.

Former President Donald Trump and Republican members of Congress argue that on President Joe Biden’s watch, the border crisis has become a major national security threat facing the U.S. Among other things, they have noted a spike in migrants from China crossing the southern border illegally since 2021, many of them military-age males. 

Some dismiss such rhetoric as racist and/or politically motivated fearmongering, arguing that terrorists and spies have the resources to try entering the country legally, while the vast majority of unauthorized migrants are driven by a hope of better socioeconomic conditions. “They’re here to chase the American dream,” says Sam Schultz, a relief worker at the California-Mexico border. “They don’t want to blow it up.”

But some top Democrats also express concern. Here we look at the evidence.

What evidence is there that terrorists may be crossing the border?

The U.S. Border Patrol is encountering a far higher number of individuals on the terrorist watch list, with the annual total increasing from single digits during the Trump administration to 172 in fiscal year 2023. That’s not just a result of increased illegal immigration; the proportion of encounters involving someone on that watch list grew more than tenfold, from 0.0007% to 0.008%, according to government data. That’s a tiny fraction of total flows, but experts point out that just a handful of people can carry out significant attacks.

This spring, Mr. Wray warned Congress of the potential for a “coordinated attack” in the U.S., similar to the one that killed 145 people at a Moscow concert hall in March. The Islamic State (ISIS) claimed responsibility for that attack. The U.S. linked it to the Afghan branch of the organization, known as ISIS Khorasan (ISIS-K), which killed 13 U.S. service members and nearly 200 Afghans at the Kabul airport during the U.S. pullout in 2021. Russia identified four suspected assailants, all from Tajikistan.

So when news emerged June 11 that U.S. authorities had arrested eight Tajiks who had crossed the southern border over the past year and had potential ISIS ties, it sparked a new round of warnings about a possible terrorist attack in the U.S. The concerns were further amplified with a subsequent NBC report that more than 400 migrants, some of whom had been deported but others whose whereabouts were still unknown, had been brought into the U.S. by an ISIS-affiliated smuggling network.

Is the threat increasing?

Historically, there is little evidence that unauthorized immigrants carry out attacks. A University of Maryland project on radicalization lists only 21 of 3,528 offenders as being an “undocumented resident.” A 2019 academic paper found that a correlation between migration and terrorism in Western Europe was driven in part by right-wing groups aggrieved by the influx. 

Until recently there was no empirical evidence that foreign terrorist groups were crossing the U.S. border. Now that is shifting, however, amid increased flows and a broader range of nationalities crossing illegally.

“Al Qaeda and their affiliates, ISIS and their affiliates, have all identified this as a vulnerability in the United States’ defense,” says Christopher O’Leary, an FBI counterterrorism veteran now serving as senior vice president of global operations with The Soufan Group. “You have massive waves of people coming across; it’s certainly reasonable to think that you could blend into that.”

Where does terrorism rank among national security concerns?

“There’s no issue I’m watching more carefully,” Democratic Sen. Mark Warner of Virginia, Senate Intelligence Committee chair, told journalists at a Monitor Breakfast on June 18. “I’m monitoring it very, very closely.”

Another congressional leader, GOP Rep. Michael McCaul of Texas, says it’s part of the broader geopolitical context.  

“When you project weakness, you get conflict and war,” says Mr. McCaul, who chairs the House Foreign Affairs Committee and previously led the Homeland Security Committee. “The last line of defense is the border.”

He points out that when the U.S. abruptly left Bagram Air Base in Afghanistan in 2021, thousands of ISIS-K prisoners were freed. And he’s concerned that conflicts in Europe, the Middle East, and the Pacific could spiral into another world war.

Part of the challenge is how to allocate U.S. resources. Mr. O’Leary, who worked on FBI counterterrorism investigations for more than two decades until stepping down last fall, says the government has pivoted away from the terrorism threat to focus on Russia, China, and great-power competition. He stresses the need to stay alert, 20-plus years into the global war on terror, with U.S.-designated terrorist groups, such as Al Qaeda and ISIS, growing. For example, Al Qaeda’s core membership increased approximately tenfold from 2001 to 2018, according to estimates.

“We know what’s going to happen if we close our eyes and turn away and hope the bogeyman is going to go away,” he says.

What is being done?

According to U.S. Customs and Border Protection, when Border Patrol agents encounter migrants crossing the southwest border illegally, they screen and vet those individuals. Agents ask for names, birthdates, and other biographical information, and take fingerprints and retinal scans. This biometric data can help establish a migrant’s identity if they use an alias or don’t have an ID. Their information is then checked against law enforcement and national security databases for “derogatory” information. 

No such information turned up during the initial screenings of the eight Tajiks. If such information comes to light later, as it did in this case, “enforcement action” will be taken accordingly, Department of Homeland Security Secretary Alejandro Mayorkas said June 26 in Tucson, Arizona. “The safety and security of the American public is indeed our highest priority.”

Reports from the department’s inspector general, however, identify gaps in screening – from Customs and Border Protection being unable to access biometric data from a Department of Defense watchlist, to not having a dedicated procedure to screen asylum applicants whose cases drag on, to obtaining but not sharing information from the FBI’s Terrorist Screening Center, which resulted in the release of a migrant on the terrorist watch list. 

Another gap is the inability to screen “gotaways,” hundreds of thousands of whom have been detected crossing by cameras and sensors, or by border agents too occupied to respond. 

So, what can be done? The president has taken executive action to stem the tide. CBS News reported today that encounters are down to their lowest level since 2021, citing preliminary data.

On the law enforcement front, the Department of Justice announced in June that a high-ranking leader of MS-13 was arrested on terrorism charges. Less than a week later, the FBI announced “the dismantlement of the largest ISIS online propaganda network and infrastructure” in the world, thanks to coordination with European partners.

A key line of defense is maintaining good ties with communities where potential terrorists may try to blend in, says Daniel Byman, senior fellow with the Warfare, Irregular Threats, and Terrorism Program at the Center for Strategic and International Studies. He notes that Muslim American communities have tipped off law enforcement to many suspected Al Qaeda and ISIS terrorists, including Omar Mateen, who later killed 49 people in the 2016 Pulse nightclub shooting in Orlando, Florida. 

“Something to think about with all this,” he says, “is when you start to demonize different migrant communities, they tend to be less willing to go to the police.”

Staff writer Francine Kiefer contributed reporting from the border at Jacumba Hot Springs, California.