The US military said on Friday that a single Islamic State bomber killed 13 US troops and at least 170 Afghans at Kabul airport last August, not the complex attack originally suspected, and that it could not have been prevented with the resources on hand.

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Briefing reporters on the results of a military inquiry, Marine General Frank McKenzie, head of US Central Command, said the bomb sent 5mm ball bearings ripping through a packed crowd at the airport’s Abbey Gate. The investigation found no definitive proof of any gunfire.

"I want to acknowledge that the investigation differs from what we initially believed on the day of the attack," McKenzie said. "At the time, the best information we had in the immediate aftermath of the attack indicated that it was a complex attack by both a suicide bomber and ISIS-K gunmen," he added.

The bombing occurred on Aug. 26 amid the evacuation of US nationals and at-risk Afghans from Kabul airport. According to McKenzie, no one was killed or injured by gunfire, adding that the explosively fired ball bearings caused wounds that looked like gunshots.

"The investigation found no definitive proof that anyone was ever hit or killed by gunfire, either U.S. or Afghan. This conclusion was based upon the careful consideration of sworn testimony of more than 100 witnesses, and especially those witnesses in observation towers, both American and British, who were in locations unaffected by the blast and that had commanding views of the scene before, during and after the explosive attack," McKenzie said.

The attack put the US military on a heightened state of alert that may have contributed to a botched US drone strike that mistook civilians for Islamic State militants.

Asked whether it could have been prevented, Army Brigadier General Lance Curtis said: "Based upon our investigation, at the tactical level, this was not preventable. And the leaders on the ground followed the proper measure."

McKenzie and other US military officials said the investigation had gathered testimony from more than a hundred witnesses, analysis from medical examiners and explosives experts, drone footage and other evidence.

Officials said the bomb was made from about 20 pounds of military-grade explosives and the bomber most likely raised it before detonating it. Beyond the 13 US servicemembers killed, some 45 others were wounded.