The Pentagon has concluded that the August suicide bombing in Kabul, Afghanistan, that killed 13 U.S. service members and more than 170 Afghans during the frantic evacuation near the U.S. embassy was "not preventable."

"Based on our investigation at the tactical level, this was not preventable," General Lance Curtis told reporters at a briefing Friday. "And the leaders on the ground followed the proper measures. And any time there was an imminent threat warning they followed the proper procedures: they lowered their profile, they sought cover and, at times, they even ceased operations at the gate."

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The probe found that the terrorist carried 20 pounds of explosives packed with ball bearings and that, despite an initial belief that it was a multi-faceted attack, it was a single attack. Gunfire that was believed to be part of the attack was later found to be warning shots fired by British or American troops.

ISIS claimed responsibility for the Aug. 26 attack, which killed 11 U.S. Marines, a sailor and a soldier. The blast occurred as service members were assisting the evacuation of thousands of Afghans who had gathered at Abbey gate in the hope of getting out of Afghanistan as the Taliban advanced ahead of the U.S. withdrawal.

U.S. soldiers stand inside the airport as hundreds of people gather near an evacuation control checkpoint on the perimeter of the Hamid Karzai International Airport, in Kabul, Afghanistan, Aug. 26, 2021.  (AP Photo/Wali Sabawoon)

The investigation found that the terrorist bypassed security checkpoints, that the Taliban likely did not know of the attack and that there was no specific intelligence about the threat itself. The blast was so brutal even those with body armor and helmets were killed in the attack. Ball bearings caused wounds that looked like gunshots.

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"The disturbing lethality of this device was confirmed by the 58 U.S. service members who were killed and wounded despite the universal wear of body armor and helmets that did stop ball bearings that impacted them but could not prevent catastrophic injuries to areas not covered," Gen. Frank McKenzie, head of U.S. Central Command, said.

Accompanying video shows a chaotic scene as civilians fled the area, while Marines and others moved in to evacuate those caught in the blast.

Officials noted that the troops were doing a dangerous job as they sought to evacuate what would eventually be about 126,000 people from the embattled country.

The Pentagon’s report comes just days after a new report by Republicans on the Senate Committee on Foreign Affairs blasted the Biden administration for its handling of the frantic withdrawal, including the decision to abandon Bagram Air Base.

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"It is clear the senior leadership of the Biden Administration failed to effectively plan, coordinate and execute an orderly withdrawal and evacuation" from Afghanistan, the report states. "The Biden Administration squandered precious time, ignored intelligence and recommendations from people on the ground, and refused bipartisan support to give them the resources to succeed. In the process, the botched withdrawal has tarnished America’s reputation and credibility."

The Associated Press contributed to this report.