FIRST ON FOX: Homeland Security Secretary Alejandro Mayorkas is facing a grilling from Congress for the Biden administration’s refusal to turn over the nationalities of people arrested on the FBI terror watchlist at the border by Border Patrol.

"I suspect that the real reason you’re unlawfully withholding this information about terror suspects’ nationalities isn’t due to privacy or security concerns, but rather partisan concerns that it would alarm the American people," Sen. Tom Cotton, R-Ark., said in a letter to Mayorkas. "Once again, you’re attempting to hide evidence of the Biden-Harris administration’s border crisis."

Customs and Border Protection (CBP) wrote to Fox News last week again denying a request to provide the nationalities of suspects on the FBI terror watchlist arrested at the southern border between ports of entry by Border Patrol.

BIDEN-HARRIS ADMIN REFUSES TO REVEAL NATIONALITIES OF TERROR WATCHLIST MIGRANTS NABBED AT BORDER

Mayorkas border

U.S. Homeland Security Secretary Alejandro Mayorkas, is interviewed by David Rubenstein, co-founder of Carlyle Group Inc., during an Economic Club of Washington event at the Marriott Marquis Hotel on May 17, 2024, in Washington, D.C. To reduce the time migrants spend waiting while their asylum claims are processed, the Biden administration will accelerate the review process as part of the latest effort to curb illegal border crossings ahead of the November election. ((Photo by Kent Nishimura/Getty Images))

The watchlist, now called the Terrorist Screening Dataset, includes known or suspected terrorists, as well as additional individuals believed to pose a potential threat to the U.S., including affiliates of individuals on the watchlist.

The request sought only the nationalities of those encountered and no further information, but it was denied. Fox made the request in October last year, and was initially denied in May, citing privacy and security concerns. The agency doubled down on that reasoning last week.

"[Customs and Border Protection (CBP)] is committed to protecting the identity of individuals and avoiding divulging information about any individual by either direct or indirect means," the agency said in a letter to Fox. "Releasing data for a particular nationality, or nationalities, that reflect a small number of individuals could lead to identification, especially by organizations familiar with the individuals."

The agency also claimed that the release of the information would reveal investigative techniques used in processing and apprehending terrorists.

BIDEN ADMIN REFUSES TO REVEAL TERROR WATCHLIST NATIONALITIES AS ILLEGAL IMMIGRATION EXPLODES ON HIS WATCH 

Cotton arrives to Homeland Security Committee meeting

Sen. Tom Cotton is slamming Kamala Harris over her refusal to be clear about her changing policy positions from 2020. (Bill Clark/CQ-Roll Call, Inc via Getty Images)

"By providing this information, it could allow targets to alter their behavior to avoid detection and exploit the gaps in CBP’s law enforcement intelligence, as well as compromise national security. Additionally, providing the requested information could disclose terrorist travel trends by geographic area which could help tip off terrorists about the government’s knowledge of travel plans, allowing the terrorists to take countermeasures against the investigators and their investigations," it says.

The agency argued that the disclosure of nationalities could allow bad actors "to undertake countermeasures to avoid CBP’s law enforcement activities and exploit any vulnerabilities in CBP’s law enforcement efforts."

Cotton, however, was not convinced by the reasoning provided.

"Of course, if the Biden-Harris administration impartially enforced our laws and deported illegal aliens regardless of their origin, terrorists wouldn’t be able to game the system in the manner you describe."

He later says that "the American people have a right to know who is crossing our border, especially when those illegal aliens have ties to terrorism."

CLICK HERE FOR MORE COVERAGE OF THE BORDER SECURITY CRISIS

"And that right, contrary to your bizarre assertion, ‘far outweighs’ the privacy rights of illegal aliens suspected of terrorism,’ he said. "Honestly I can’t believe I even have to write that sentence."

The letter requests the information by Sept. 10.

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There were 172 encounters of nationals on the terror watchlist at the border between ports of entry last fiscal year and more than 560 at the ports of entry.

Fox News' Bill Melugin contributed to this report.