As the Taliban took control of Kabul, the capital of Afghanistan, on Sunday, the terrorist group also seized Bagram Air Base, freeing thousands of U.S. prisoners, including some of the Taliban's most hardened fighters and senior Al Qaeda operatives.

U.S. forces handed control of the airfield, located on the outskirts of Kabul, over to the Afghan government in early July. Afghan forces at the base surrendered to the Taliban on Sunday, as other Taliban troops stormed the U.S. Embassy in Kabul and the presidential palace.

Afghan officials estimated that the U.S. left about 5,000 to 7,000 prisoners in the air base when the Americans left, according to Axios.

The Pul-e-Charkhi prison, the largest in Afghanistan, contained a maximum-security cell block for Al Qaeda and Taliban prisoners. Afghan government troops surrendered control of the base without a fight.

Chairman of the Joint Chiefs of Staff Gen. Mark Milley told senators that the freed prisoners will speed up a previous assessment of how quickly terrorist groups will reconstitute themselves in Afghanistan, Axios reported.

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Sen. Lindsey Graham, R-S.C., asked Milley and Secretary of Defense Lloyd Austin whether they would revise a June assessment they sent to Congress warning of a "medium" risk of terrorist groups reconstituting themselves in the country within two years. 

"Yes," Milley said. He told Graham that he would have to assume the terrorists would return to their full strength more quickly. He offered to brief the senators in a classified setting.