Thousands of "special interest aliens" from numerous countries, including the Middle East, have been arrested by Border Patrol agents while attempting to cross the U.S. southern border illegally over the last two years, according to internal Customs and Border Protection (CBP) data leaked to Fox News.

"Special interest aliens" are people from countries identified by the U.S. government as having conditions that promote or protect terrorism or potentially pose some sort of national security threat to the U.S.

That data, confirmed by multiple CBP sources and reflects apprehensions between ports of entry between October 2021 and October 2023, shows that agents encountered 6,386 nationals from Afghanistan in that period as well as 3,153 from Egypt, 659 from Iran and 538 from Syria.

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Agents also encountered 13,624 from Uzbekistan, 30,830 from Turkey, 1,613 from Pakistan, 164 from Lebanon, 185 from Jordan, 139 from Yemen, 123 from Iraq and 15,594 from Mauritania. The data does not include information on how many of those migrants were removed or who were released into the U.S. with a court date.

Border patrol truck at border fence

A Border Patrol agent walks between a gap along the border wall between the U.S. and Mexico in Yuma, Arizona, on June 1, 2022. (Patrick T. Fallon / AFP via Getty Images)

Those numbers do not include encounters by CBP’s Office of Field Operations at ports of entry. It also does not include the numbers who have snuck past agents without detection -- sources say there have been over 1.5 million such "gotaways" during the Biden administration.

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Meanwhile, fiscal year 2023 broke the record for encounters on the FBI terror watch list with 151 people encountered at the southern border between ports of entry, higher than the previous six years combined.

Border Patrol sources tell Fox they have extreme concerns about the people coming across from special interest countries, given they have little to no way to vet them. Unless they have committed a crime in the U.S. or are on a federal watch list, agents have no way of knowing their criminal history because their countries do not share data with the U.S., so there is nothing to match their name against when authorities run their fingerprints.

Mexico Migrants Kidnapped

Migrants stand on the Mexican side of the U.S.-Mexico border on the banks of the Rio Grande in Ciudad Juarez on March 29, 2023. (AP Photo / Fernando Llano / File)

The data comes as the U.S. is struggling to deal with a new wave of migrants at the southern border, with numbers again hitting historic highs. Sources have told Fox that there were more than 260,000 encounters in September, which marks a new monthly record. Republicans in particular have raised concerns about the potential security threat of an environment in which Border Patrol agents are overwhelmed and migrants are being processed in significant numbers into the U.S. interior.

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The Department of Homeland Security’s threat assessment, published last month, noted that agents have encountered a growing number on the watch list and warned that "terrorists and criminal actors may exploit the elevated flow and increasingly complex security environment to enter the United States."

"Individuals with terrorism connections are interested in using established travel routes and permissive environments to facilitate access to the United States," the assessment also said.

Last week, the Biden administration moved to waive more than two dozen federal regulations in order to build a border wall in South Texas, citing an "acute and immediate need" in order to prevent unlawful entries. However, the administration has since distanced itself from the move, noting that it was funding appropriated in 2019 and which Congress has refused to divert to other projects.