The investigation ruled out more than one perpetrator or anyone using firearms in the August 26 attack, which was claimed by the Islamic State group.

At least 160 Afghan civilians and the 13 US troops were killed by the bombing, which came during the final days of the US military's withdrawal after two decades of war, according to the investigation.

The bomb exploded in a dense crowd just outside the airport's Abbey Gate as thousands of people pushed to try to get inside and leave the country in the US-managed airlift.

Although some gunfire erupted after the bombing, US officers said they were warning shots and none of those who died in the event were killed by them.

"There were no gunshot wounds" among the victims, said Brigadier General Lance Curtis, who presented the investigation findings Friday.

He said that the deaths were from shrapnel including ball bearings from the bomb, the wounds of which can look like gunshot wounds.

Curtis admitted that on the day, the US military thought the attack was "complex," involving an Islamic State gunman as well as the bomber.

"We now know that the explosive fired ball bearings causing wounds that looked like gunshots. When combined with a small number of warning shots, that led many to assume that a complex attack had occurred," Curtis told reporters.

Also adding to the confusion was the fact that the shrapnel from the blast punctured tear gas canisters carried by the US troops for crowd control.

That created "instant chaos and sensory overload," said one of the officials who briefed reporters on the investigation.

The bomb also left 45 US service members injured, some with brain injuries from the concussive force of the blast.

The black-garbed perpetrator, shown in the investigator's sole video of the bombing, was later identified by Islamic State as Abdul Rahman Al-Logari, who was released from a government prison by the Taliban after they took control of Kabul on August 15.