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FIRST ON FOX: A top House Republican is warning of an "urgent" terrorist threat to the United States after the Taliban's release of thousands of prisoners from Bagram Air Base in Afghanistan, including the ISIS-K suicide bomber that killed 13 U.S. service members, suggesting that those released prisoners could attempt to enter the country through the "porous southern border," and demanding answers from the Biden administration on its plans to quell that threat.

Ranking Member of the House Subcommittee on Immigration and Citizenship Rep. Tom McClintock, R-Calif., wrote a letter to Homeland Security Secretary Alejandro Mayorkas on Thursday, warning that released prisoners could attempt entry to the U.S. through Mexico with the purpose of committing terrorist attacks on the homeland. 

ISIS-K SUICIDE BOMBER WAS PRISONER RELEASED FROM BAGRAM AIR BASE, SOURCES SAY

"I write with grave concern over the security risks posed by the release of 7,000 terrorists, including members of the Taliban, ISIS-K, and Al-Qaeda from Bagram Air Base combined with the porous southern border," McClintock wrote. "Shortly after the president ordered the unconditional surrender of Bagram Air Base to the Taliban, Taliban forces took control and released the terrorists that had been in U.S. custody. Days later, 13 U.S. service members were murdered by a suicide bomber at Hamid Karzai International Airport in Kabul." 

Citing Fox News reporting, McClintock wrote that Biden administration officials "have confirmed that the suicide bomber ‘was a prisoner released from Bagram Air Base on August 15 after the Taliban seized control of the facility.’" 

Taliban fighters escort women marching in support of the Taliban government outside Kabul University, Afghanistan.  (AP Photo/Bernat Armangue)

The ISIS-K suicide bomber who killed 13 U.S. service members outside of Hamid Karzai International Airport in August was a prisoner released from Bagram Air Base on Aug. 15 after the Taliban seized control of the facility, officials confirmed to Fox News. 

Fox News first reported that Rep. Ken Calvert, R-Calif., the top Republican on the House Appropriations Subcommittee on Defense, said U.S. and Indian intelligence sources reported to him that the suicide bomber was one of the 7,000 prisoners housed at Bagram prison and released by the Taliban in August. 

"We know that one of those terrorists exploded the bomb that killed 13 U.S. Marines on August 26, but the whereabouts of the others is unknown," McClintock wrote. "It is possible, if not likely, that many will attempt to enter our country through Mexico with the purpose of committing similar terrorist attacks." 

The Aug. 26 suicide bombing took the lives of 13 U.S. service members – including 11 Marines, one Navy sailor and one Army soldier. Eighteen other U.S. service members were wounded. The bombing also left more than 150 civilians dead.

"In fact, we already know that aliens found in the Terrorist Screening Database have been apprehended on the southern border this year," McClintock wrote. "The release of thousands of terrorists back into Afghanistan and the criminal cartels' control of human smuggling on the southwest U.S. border, mean the terrorist threat is urgent."

TOP REPUBLICAN SAYS ISIS SUICIDE BOMBER WAS PRISONER RELEASED FROM BAGRAM AIR BASE, CITES INDIAN INTEL

All U.S. forces were removed from Bagram, the largest military base in Afghanistan, in July. 

The Pentagon said on Aug. 27 "thousands" of ISIS-K prisoners were freed by the Taliban during their takeover of Afghanistan in the days leading up to the bombing near the Kabul airport. 

As the Biden administration began the withdrawal of military assets, provincial capitals across Afghanistan began to fall to the Taliban. By mid-August, the Taliban attained control of two-thirds of Afghanistan. And by the time the U.S. withdrew all U.S. troops from the country on Aug. 31, Kabul had also fallen to the Taliban. In mid-August, U.S. intelligence assessments projected the capital city could fall to the Taliban within 90 days. 

Meanwhile, in his letter, McClintock demanded Mayorkas provide information to "help us understand the ongoing terrorism-related threats at the southwest U.S. border and how the Biden administration's failures in Afghanistan affect border security." 

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McClintock gave Mayorkas until Oct. 28 to provide: a list of countries whose nationals have been encountered on the southwest border by U.S. Border Patrol during 2021; the number of individuals from each country; the nationalities of each individual encountered at the border who had a Terrorist Screening Database "alert" from 2015 to 2021; and a written notification of each instance in which an individual is encountered by DHS officials whether at a port of entry, or between ports of entry, and whether they were previously imprisoned at Bagram Air Base. 

DHS did not immediately respond to Fox News' request for comment. 

McClintock's letter comes as the Biden administration continues to grapple with a migrant crisis along the U.S. Southern border. 

There were more than 200,000 migrant apprehensions in the months of July and August, which has fueled criticism of the administration's rolling-back of border protections like the Migrant Protection Protocols (MPP) – which kept migrants in Mexico as they went through immigration proceedings. The administration was ordered to reinstate the policy by a federal order and has since announced it will seek to end the policy in line with the ruling.

Fox News' Adam Shaw contributed to this report.