Top Senate Republicans have pushed the Biden administration’s top border official to explain how federal law enforcement allowed eight suspected ISIS terrorists into the United States at the U.S.-Mexico border.

Sen. Lindsey Graham (R-SC), the top Republican on the Senate Judiciary Committee, and Sen. Rand Paul (R-KY), the top GOP member on the Senate Homeland Security and Governmental Affairs Committee, announced on Monday that they had contacted Homeland Security Secretary Alejandro Mayorkas and demanded an explanation for how the department screened and vetted eight Russian citizens from Tajikistan.

“The circumstances of these arrests have highlighted significant potential vulnerabilities within our border security and immigration screening processes,” Graham and Paul wrote in the June 14 letter.

“Given the serious national security implications of these deficiencies, it is imperative that we receive detailed information to fully understand the scope of these issues and to ensure that our border infrastructure and national security systems are adequately safeguarding against such threats,” the letter stated.

Senate Republicans erupted with fury last week following the publication of a New York Post report that stated U.S. Immigration and Customs Enforcement had arrested the immigrants during busts in Los Angeles, New York City, and Philadelphia.

Authorities wiretapped individuals and recorded a conversation about making bombs.

The suspects were encountered at the southern border and released into the country. U.S. Customs and Border Protection did not realize until after their release, with at least two occurring in 2023, that they posed a serious national security threat despite the Biden administration’s assurances that all immigrants arrested were adequately screened and vetted before being let into the country, according to the outlet.

Graham and Paul asked Mayorkas to turn over all “A-Files,” the name of the document that the department uses to track immigrants once they are apprehended at the border and released into the country to await court proceedings.

The senators also demanded Mayorkas to send over all communications between the department and the FBI, as well as internal communications about the eight suspected terrorists, details about each person’s arrest and detainment, addresses they provided at the time of release from custody, and what criminal and international databases revealed of the immigrants’ histories at the time that they were checked against those systems.

Mayorkas has until June 28 to provide the senators with the requested information.

The number of illegal immigrants apprehended at the southern border and determined while in custody to be on the FBI terrorist watch list has spiked under President Joe Biden.

In fiscal 2023, which ended in September last year, the Border Patrol caught more than 172 known or suspected terrorists illegally entering the U.S. from Mexico or Canada.

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The figure is a far cry from single-digit terrorist watch list numbers seen throughout the Trump administration. The dramatic increase to double and now triple digits over the past several years has especially drawn concern from Democratic and Republican lawmakers.

FBI Director Christopher Wray testified before the Senate in August 2022 that although there was no “imminent threat from a foreign terrorist organization on the border at the moment,” terrorists were looking for any vulnerability to “exploit.”