Roughly a dozen individuals who are not US citizens and are on the terror watchlist were encountered by federal officials at the US southern border from October 2017 to October 2018, according to an administration official familiar with data from Customs and Border Protection.
The number of individuals encountered at the southern border is a very small percentage of the total known or suspected terrorists who tried to enter or travel to the US in fiscal year 2017. That much larger number has been touted by the administration as it seeks to gain support to build a wall on the border.
The official adds there are not significant numbers of known or suspected terrorists crossing the southern border but the number went from "zero to a small increase" over the last couple of years.
But the official said that while the number of potential terrorists trying to cross the border is minimal, the Department of Homeland Security is concerned that terrorists could try to exploit immigration patterns.
It was not immediately clear how many US citizens who are on the terror watchlist were encountered at the border in the same period.
A State Department report for the year 2016 said, "There are no known international terrorist organizations operating in Mexico, no evidence that any terrorist group has targeted U.S. citizens in Mexican territory, and no credible information that any member of a terrorist group has traveled through Mexico to gain access to the United States."
Of the approximately dozen individuals, around half were prevented from entering the country at a legal port of entry on the southern border and the other half were apprehended crossing the border illegally between ports of entry.
The official did not provide details of whether any of the individuals are currently in US custody. The official noted that just because someone is believed to have a tertiary affiliation doesn't mean there is a prosecutable crime for the Department of Justice to pursue, but it's enough to make sure the individual doesn't make it into the US and for the US to pursue repatriation.
DHS has said -- and reiterated in a fact sheet released Monday night -- that 3,755 known or suspected terrorists tried to enter or travel to the US in fiscal year 2017. But those numbers are for all entry points and visa applications around the world, not just at the southern border.
CNN has reported that the number is misleading when provided in the context of the southern border, as it primarily reflects individuals who were blocked from entering the US when they applied for visas or sought to travel to the US, including by air.
Both the official and DHS also distinguish between individuals on the terror watchlist and what the department calls "special interest aliens," who come from hostile countries or ones with terrorist activity and take irregular routes to the southern border.
Nielsen said some 3,000 "special interest aliens" came to the southern border last year.
CLARIFICATION: This story has been updated to reflect that the individuals encountered at the border on the terror watchlist were non-US citizens.