Making sense of GOP claims about who is crossing the border

In recent days, as the Biden administration faces a new surge of migrants along the southwest border, Republican lawmakers are renewing warnings that international terrorists are trying to slip into the country through Mexico. But U.S. officials and experts say such warnings don't accurately capture how unlikely it is that international terrorists could access the United States through the U.S.-Mexico border.

"The notion that the Southwest border is open to terrorists is ludicrous," said Alex Nowrasteh, the director of immigration studies at the Cato Institute. "The government has shown no evidence that terrorists use the Southwest border to get into the United States."

Speaking before reporters and cameras during a visit to the border in Texas on Monday, House Minority Leader Kevin McCarthy, R-Calif., disclosed that border patrol agents had just told him that suspected terrorists were recently apprehended trying to enter the United States.

"You saw in their eyes, they're on the list ... the terrorist watch list," McCarthy said.

"They are now finding people from Yemen and Iran and Turkey, people on the terrorist watch list," he added.

Other Republicans offered similar warnings.

"Individuals that they have on the watchlist for terrorism are now starting to exploit the southern border," Rep. John Katko, R-N.Y., said at the news conference with McCarthy and a dozen others.

Specifically, on Jan. 28, a Serbian national apprehended at the border in New Mexico was a positive match for someone on the U.S. government's Terrorist Screening Database, or TSDB, according to a source familiar with the matter. In addition, three others on the database -- all from Yemen -- were recently caught at the border, the source said.

But showing up on a terror watchlist doesn't mean someone is necessarily a terrorist, and it's exceedingly rare for someone encountered by a Border Patrol agent or a customs officer to be on that terrorist watchlist.

"Less than 0.0001 percent of total [Customs and Border Protection] inadmissible encounters and apprehensions involve persons who are watchlisted on the Terrorist Screening Database (TSDB)," a senior Homeland Security Investigations official told Congress in 2018.

"It's an alert, it doesn't mean you arrest that person on site and charge them," David Lapan, a spokesman for the Department of Homeland Security under President Donald Trump, said of the watchlist. "There are many different ways that people end up on the terror watchlist. Sometimes it's a mistaken identity."

Others end up on the watchlist simply because they are "a cousin or a friend or a house cleaner" to a known terrorist, one U.S. official, speaking on the condition of anonymity, noted.

Furthermore, the U.S. government has established systems to prevent terrorists from entering the country -- as exhibited by the fact that four people on the government watchlist were successfully intercepted in recent months, Homeland Security Secretary Alejandro Mayorkas suggested to lawmakers during a House hearing on Wednesday.

Terrorists trying to sneak into the United States is "not a new phenomenon," but "it is because of our multi-layered security apparatus [and] architecture that we have built since the commencement of the Department of Homeland Security that we are in fact able to identify and apprehend them," Mayorkas said. "We actually deny them entry based on our intelligence and based on our vetting procedures, which have only grown in sophistication over the years."

As for Nowrasteh, the director of immigration studies at the Cato Institute, he studied the topic in a report on the risk of foreign-born terrorists in the U.S. between 1975 and 2017. In that time period, nine illegal immigrants of the more than 31 million people who entered the country illegally became terrorists – including three Macedonian brothers who entered the country as children and were raised in the United States.

"There are more people in the country illegally that overstay their visas than come across the southwest border," Lapan told ABC News, referencing the 9/11 hijackers, who all entered the United States on airplanes and overstayed their visas.

"It's not that there's zero chance of that happening, but we've just never seen evidence that it's a significant risk," he said.

In fact, "There has never been a terrorist attack in the United States by someone who illegally crossed the southern border," the U.S. official added.