Washington — Concerns about whether known or suspected terrorists are exploiting the migration crisis along the U.S.-Mexico border to enter the country have intensified following the brutal terrorist attacks carried out by Hamas in Israel over the weekend.

Republican lawmakers, GOP White House hopefuls and conservative media figures have argued that the Biden administration's border policies have given terrorists an easier way to enter the U.S. and harm Americans. On Monday, former President Donald Trump claimed that the "same people" who killed or abducted more than 1,000 civilians in Israel are coming across the southern border separating the U.S. and Mexico, offering no evidence to support his assertion.

There has been a marked increase in Border Patrol apprehensions of individuals with matches on the U.S. terror watchlist over the past two years. But they represent a tiny fraction of all migrants processed along the southern border. Such incidents are more common along the U.S.-Canada border, and not all those on the watchlist are suspected terrorists. 

Still, there are valid concerns about whether the U.S. has sufficient tools to ensure it detains all national security threats, including those entering the country clandestinely. 

"Are terrorists flooding across the border? Probably not," said Theresa Cardinal Brown, a former Department of Homeland Security official under Presidents George W. Bush and Barack Obama. "But at the same time, it is true that the large number of people arriving does have national security implications."

Here's what we know about this issue, based on government data, reports and policy:

A spike in terror watchlist hits along the U.S.-Mexico border

When Border Patrol apprehends individuals, it is supposed to run criminal and national security screenings on them. The process includes checking names against the Terrorist Screening Data Set, or TSDS, an FBI system that tracks known or suspected terrorists as well as their affiliates. 

Border Patrol apprehensions of individuals on the FBI's terrorism watchlist have increased sharply in recent years as the number of overall crossings recorded by the agency along the U.S.-Mexico border has soared to record levels.

In fiscal year 2023, Border Patrol reported apprehending 151 migrants with positive terrorism watchlist matches who entered the U.S. illegally along the southern border, an all-time high for the region that eclipsed the previous record of 98 set in fiscal year 2022, government figures show. In fiscal year 2021, the agency reported just 15 such apprehensions.

When including those processed at official ports of entry, there were 227 terror database hits with individuals processed along the southern border in fiscal year 2023.

Have any Hamas fighters been caught crossing the border?

There have been no indications that any individuals affiliated with Hamas have been arrested along the U.S.-Mexico border in recent years. However, the available data on migrants who appear on the terror watchlist does not show who they are, where they hail from or the group they are associated with.

So far, the U.S. has not publicly reported recent incidents of Border Patrol arresting terrorists with connections to al Qaeda or ISIS, either. Earlier this year, however, U.S. officials did report a case in which a smuggler with ties to ISIS helped migrants from Uzbekistan travel to the southern border. The White House said there were no indications the migrants had terrorist links or were plotting an attack.

In a statement this week, DHS said it is "prepared to respond" to potential threats to the homeland stemming from the attacks in Israel and if necessary, to alter its "security posture."

"While we do not have specific and credible intelligence indicating a threat to the United States at this time stemming from the Hamas terrorist attacks in Israel, DHS is closely monitoring unfolding events and will continue to engage in information sharing with our intelligence and law enforcement partners at home and abroad," the department added in its statement.

A watchlist hit does not mean the person is a terrorist

If there's a terror database hit, it does not necessarily mean the person is a known or suspected terrorist. They could be. But they could also be the relatives or friends of suspected or known terrorists, or someone linked to an organization no longer considered a terrorist group. It could also be a faulty hit.

"Inclusion in the TSDS ranges from known associates of watchlisted individuals, such as family members, to individuals directly engaged in terrorist activity," a 2023 DHS report said.

Cardinal Brown, the former DHS official who now works as a senior adviser for the Bipartisan Policy Center, said the database is "very broad." Not everyone on it, she said, "wants to do the United States harm."

"TSDS encounters may include encounters with family members or associates of a known or suspected terrorist who after evaluation, in coordination with the FBI, may not present a danger to the United States," DHS said in another report this year.

Watchlist hits may not involve the terrorists you often think of

In the U.S., international terrorism is often associated with high-profile Islamic fundamentalist groups like al Qaeda, ISIS and Hamas that mainly operate in the Middle East and Africa.

However, the vast majority of migrants processed at the southern border are from the Western Hemisphere. Some migrants with terror database hits in recent years have hailed from Colombia and Venezuela, and may have associations with domestic guerilla fighters — not international terrorist organizations — that are or were previously designated as terrorist groups by the U.S.

For example, the U.S. has delisted Colombia's demobilized leftwing guerillas and rightwing paramilitaries — including the Revolutionary Armed Forces of Colombia, or FARC — from its list of foreign terrorist groups. But people associated with these groups could still be on the terrorism watchlist. In fact, DHS officials have said not all migrants with database matches are members of active terrorist groups. The U.S. still considers guerillas who have not demobilized in Colombia to be part of terrorist groups.

A database hit does not mean the person was let into the U.S.

When Border Patrol apprehends someone, it has to make a decision about whether to deport, release or transfer them to another federal agency.

While many migrants and asylum-seekers are released from border custody with a court notice or other paperwork, those with matches on the FBI terror watchlist are typically detained while the government determines whether the match is accurate. The government can subsequently seek their deportation while they're in custody or transfer them to another agency for prosecution or further vetting, DHS officials said.

Those considered to threaten national security, including terrorists and spies, are a priority for arrest and deportation under the Biden administration's immigration enforcement priorities.

These cases are a tiny fraction of all migrants

While terrorism watchlist hits have increased along the southern border, they still represent a very small fraction — fewer than 0.01% — of the hundreds of thousands of migrants who cross into the U.S. illegally each year.

While Border Patrol apprehensions of individuals on the FBI terror watchlist increased to 151 along the southern border in fiscal year 2023, the agency recorded over 2 million migrant apprehensions there during that time span.

Most terror watchlist hits still occur at the northern border

Encounters of individuals with FBI terror database matches are much more common along the border separating the U.S. and Canada, despite the much lower levels of unauthorized migration there.

For example, Customs and Border Protection recorded more than 430 watchlist hits along the northern border in fiscal year 2023, the vast majority of them at official ports of entry. 

Still, there are some concerns

In its homeland threat assessment for 2024, the intelligence branch of DHS said the "record encounters of migrants arriving from a growing number of countries have complicated border and immigration security," citing the increase in terror database matches.

The assessment also said a recent increase in apprehensions of migrants from the Eastern Hemisphere, while still significantly lower than those from the Western Hemisphere, has "exacerbated border security challenges" because those individuals require more vetting and processing and because it's more difficult to deport them.

Moreover, DHS' Office of Inspector General in July published a report that raised questions about Border Patrol's system to determine whether a migrant could be a national security risk, citing the release of a Colombian migrant in 2022 who was later found to be on the FBI's terrorist watchlist. DHS pushed back on the report, noting the migrant was re-arrested once it became clear that he was on the database.

Some officials and lawmakers have also voiced concerns about whether any of the estimated hundreds of thousands of migrants who evaded apprehension after crossing the U.S. southern border unlawfully could have ties to terrorism. Border Patrol estimates more than 1 million individuals entered the country surreptitiously over the past two years.

Camilo Montoya-Galvez

Camilo Montoya-Galvez is the immigration reporter at CBS News. Based in Washington, he covers immigration policy and politics.

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