A single Islamic State suicide bomber carried out the attack at Kabul’s international airport in August that killed 13 U.S. troops and as many as 170 civilians, and was not accompanied by accomplices firing into the crowd, according to a Pentagon report released on Friday.

The findings by a team of Army investigators contradict initial reports by senior U.S. commanders that militants fired into the large crowd of civilians at the airport seeking to flee the Afghan capital and caused some of the casualties.

The report also absolved Marines of firing lethal shots into the crowd at the Abbey Gate entrance to the airport as some officials had suspected because of the large amount of ammunition the Marines fired after the attack, which took place on Aug. 26.

“The investigation found no definitive proof that anyone was ever hit or killed by gunfire, either U.S. or Afghan,” Gen. Kenneth F. McKenzie Jr., the head of the Central Command, told reporters in a video conference from his headquarters in Tampa, Fla.

General McKenzie said that conclusion was based on eyewitness testimony, medical examiners’ findings and video footage from an MQ-9 drone that flew overhead minutes after the blast. Military investigators later acknowledged that they did not speak to any Afghans because U.S. troops had left the country by the time their inquiry started.

General McKenzie acknowledged that the report contradicted commanders’ initial assessments, noting that “ball bearings caused wounds that looked like gunshots.” He added that “the battlefield is a confusing and contradictory place, and it gets more confusing the closer you are to the actual act.”

The investigation also found that military leaders on the ground took appropriate measures to protect their forces throughout the operation at Abbey Gate, and that the medical services available saved every life they possibly could have.

The Abbey Gate bombing changed the complexion of the American withdrawal from Afghanistan, sparking a tragic chain of events that ended with a botched American reprisal strike that killed 10 innocent people, including seven children.

The suicide bombing capped 20 years of war. Thirteen flag-draped coffins were flown to Dover Air Force Base in Delaware, and a succession of funerals were held across the country for U.S. service members, most of them under the age of 25.

“I’ve never been one for politics and I’m not going to start now,” Marilyn Soviak, the sister of Maxton Soviak, a Navy corpsman from Ohio who was among the dead, posted on Instagram after the attack. “What I will say is that my beautiful, intelligent, beat-to-the-sound of his own drum, annoying, charming baby brother was killed yesterday helping to save lives.”