Airport security had closed two of the gates, but decided to leave Abbey Gate open, U.S. officials said.
They also said that, earlier in the day, Taliban commanders and fighters manning checkpoints along the airport route twice pushed back surging crowds, but they came back again.
That third time, someone else came with them. At 5:48 p.m., the bomber, wearing a 25-pound explosive vest under clothing, walked up to the group of Americans who were frisking people hoping to enter the complex.
He waited, officials said, until just before he was about to be searched by the American troops, and then he detonated the bomb.
The bomb was unusually large for a suicide vest, killing himself and igniting an attack that would leave dozens of people dead, at least 170 civilians and soldiers.
At 11 kilograms, the vest worn by the suicide bomber did untold damage. According to Army manuals, suicide bombers typically wear either a belt containing four kilograms or less of explosives, or a vest packed with four to nine kilograms of explosives.
Just after the bomb went off, U.S. Defense Department officials said, fighters nearby began firing weapons.
The officials said that some of the Americans and Afghans at Abbey Gate might have been hit by that gunfire.
There was so much confusion in the aftermath of the explosion that the military initially reported that a second suicide bombing had taken place at nearby Baron Hotel.
That turned out to be false, according to Maj. Gen. Hank Taylor, the Joint Staff deputy director for regional operations.
U.S. military forces conducted their first reprisal strike in Afghanistan since the attack on Friday.
“An over-the-horizon counterterrorism operation” in the Nangarhar Province “against an ISIS-K planner,” said Capt. Bill Urban, a spokesman for U.S. Central Command, referring to the Afghan affiliate of the Islamic State that claimed responsibility for Thursday’s attack.
“Initial indications are that we killed the target,” he said. “We know of no civilian casualties.”
Breeding ground for terrorists
With the Taliban’s return to power after two decades underground, counterterrorism experts have feared that Afghanistan will become a fertile environment for terrorist groups, notably Al Qaeda and the Islamic State.
Both the Islamic State and Al Qaeda remain a potent threat in the country, terrorism experts say, despite having their numbers ground down by years of military action by the United States and a range of partners.
Yet the two are bitter rivals and operate in different ways. Al Qaeda has changed substantially since Osama bin Laden oversaw the organization and plotted the Sept. 11, 2001, terrorist attacks.
In the years since, the stature of its central leadership has declined while local militant groups in Syria, Iraq, West Africa and parts of Asia have adapted, sometimes jettisoning Al Qaeda ideology in pursuit of local goals.
The Islamic State, which itself defected from Al Qaeda, has maintained a more centralized leadership, with its local branches maintaining not just the ideology of the original organization, but also strong operational links to it.