The investigation, details of which were presented to the media Friday, was ordered after a member of Islamic State-Khorasan, the extremist group’s affiliate in Afghanistan, detonated a suicide vest Aug. 26 in a crowded outdoor corridor just outside Hamid Karzai International Airport, killing an estimated 170 Afghans and 13 U.S. troops. The attack on the airfield’s Abbey Gate occurred as the U.S. military raced to evacuate thousands of Afghan allies during a frantic, two-week rescue operation set in motion when the Taliban seized control of the capital.

Marine Gen. Kenneth “Frank” McKenzie, chief of U.S. Central Command, acknowledged Friday that the investigation contradicted statements he made in the immediate aftermath of the explosion. At the time, defense officials believed there was one, and possibly two, explosions and that gunmen opened fire on the crowd after.

That the investigation has “contradicted our first impression” was a testament to the independence of those who conducted it, McKenzie said. “It also confirms the age-old fact that the battlefield is a confusing and contradictory place, and it gets more confusing the closer you are to the actual action.”

The airport bombing was one of two high-profile incidents during the evacuation for which U.S. officials were criticized after it became apparent they had distributed inaccurate information about what transpired. The other occurred three days later, when a U.S. drone strike killed 10 civilians, including seven children – an attack senior military officials initially defended as “righteous,” saying they had killed a suspected Islamic State-Khorasan bomber preparing to hit airport. The target, in fact, was an aid worker, and several family members also perished in the attack.

Army investigators determined the explosion at Abbey Gate ejected 5-millimeter ball bearings that tore into the crowd at high velocity. Forty-five U.S. troops were wounded in the blast, with some suffering brain injuries that surfaced later, defense officials said.

The bombing set off chaos inside the airfield’s outer perimeter, where Afghans clamoring to secure seats on U.S. evacuation flights had crowded.

Defense officials said Taliban foot soldiers guarding initial checkpoints made it difficult and dangerous for potential evacuees to access the airfield, forcing many to seek alternative routes inside with the assistance of U.S. personnel.

A sewage canal along the airfield’s perimeter had acted as a natural barrier early on, aiding troops with crowd control, said Army Brig. Gen. Lance Curtis, who led the investigation. But that “changed dramatically and rapidly” as more people began taking alternative paths into the airport, he added.

Officials said they believe that the Islamic State bomber, carrying 20 pounds of explosives, bypassed the Taliban checkpoint and that the Taliban had no advance knowledge of the attack.

In the moments leading up to the attack, hundreds were packed into and along the edges of the sewage canal, with Marines standing on its inner wall, spotting and pulling people from the crowd, officials said. Had U.S. personnel tried to push outward, to fortify the perimeter as the crowd surged, the Americans would have been at greater risk, Curtis said.

Investigators, he said, had concluded the attack “was not preventable.”

Disoriented service members who rushed to aid the wounded encountered tear gas – unleashed from ruptured canisters the Marines carried on their body armor – and there was initial confusion over whether the Taliban had been involved and whether the attack was still ongoing, the investigation found.

It was initially reported that some Marines and British troops opened fire in the confusion after the explosion, believing they still could be under attack. The investigation found that two British troops and one U.S. service member fired only warning shots to disperse the crowd, while one American shot at a “suspicious individual” in a water tower outside the airport perimeter. Investigators said there was no indication anyone was hit with gunfire during or after the incident.

Curtis said the gunfire caused an echo as it traveled through the confined space, creating the illusion of a firefight. Further adding to the confusion, wounds resulting from the ball bearings resembled those caused by gunshots, officials said.

The military’s inquiry lasted more than three months. It spanned five countries and involved interviews with more than 130 people, officials said. Although U.S. officials were in routine contact with the Taliban leading up to the attack and in its aftermath, investigators did not interview any Afghan witnesses.