Pentagon: Deadly Afghan airport attack was not preventable

At the Pentagon on Friday, military officials laid out a detailed and graphic minute-by-minute account of the bombing. The bottom line, they said, was that those who died had wounds that were "so catastrophic" that they couldn't be overcome. And they said that earlier thoughts that it was a complex attack involving gunfire turned out to be unfounded.

"A single, explosive device killed at least 170 Afghan civilians and 13 U.S. servicemembers by explosively directing ball bearings through a packed crowd and into our men and women at Abbey gate," said Gen. Frank McKenzie, head of U.S. Central Command. "The disturbing lethality of this device was confirmed by the 58 U.S. servicemembers 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."

Investigators said the bomber likely got near the gate by bypassing Taliban and other security checkpoints. They said it appears the Taliban didn't know of the attack, that security precautions were being taken and that intelligence about potential threats that was circulating that day was not specific.

"Based upon our investigation, at the tactical level this was not preventable," said Brig. Gen. Lance Curtis, who led the investigation. He added that military leaders on the ground in Kabul followed proper security measures, at times closing the gate or pausing the processing of evacuees.

Military officials said that gunfire after the blast was found to be warning shots fired by U.S. and British troops, and that no one was killed or wounded by gunshots.

McKenzie said the investigation revealed that the ball bearings caused wounds that looked like gunshots. He said some troops in the area fired a number of warning shots, and that led others to believe that the attack also included gunmen.

U.S. military officials have said all along that the U.S. forces at the gates were doing a dangerous job. Overall the U.S.-led coalition evacuated about 126,000 people in about three weeks, and the U.S. left Afghanistan for the last time at about midnight on Aug. 30.

Early on, McKenzie said troops at the gates had to get close to the people they were screening.

"This is close up work. The breath of the person you are searching is upon you," McKenzie said in August soon after the attack.