On Sept. 11, 2001, an employee working the ticket counter at Dulles International Airport checked two passengers onto their flight. He didn’t know those two men would soon hijack the flight and crash it into the Pentagon, and he certainly didn’t know how much guilt he would carry for years to come.

It was American Airlines Flight 77, and according to , the two men arrived late to check in. The man who checked them in, Vaughn Allex, recently shared the whole experience nonprofit organization . Though he was just following his daily routine, Allex said he “didn’t know what [he] had done” until the next day.

The plane crashed into the Pentagon at 460 mph not long after it left the airport that morning, according to . The crash into total, with 64 being passengers on the airplane. Allex recalled the people he checked into the flight that morning—a retiree and his wife, a student group with parents and teachers.

“I came to work [the next day] and people wouldn’t look at me in the eye,” Allex told StoryCorps. “They handed me the manifest for the flight. I just stared at it for a second and then I looked up. I go, ‘I did it, didn’t I?’

“... and they were gone. They were just all gone.”

Allex was simply doing his job, but he’s felt the guilt about doing so for the past 15 years. The hijackers found loopholes in U.S. airport security long before the strict regulations of the modern day set in, and the a few months later. A story from a couple of weeks after the attacks details just how easy it was to get onto the airplanes:

At almost every step along the way, the system posed no challenge to the terrorists—not to their ability to purchase tickets, to pass security checkpoints while carrying knives and cutting implements nor to board aircraft.

The system worked the way it was intended, according to all the available evidence. For three decades, it has been preoccupied with looking for guns and explosives rather than for dangerous people. That, security experts and aviation professionals say, was its vulnerability. The terrorists did not breach the nation’s airport security system; they slipped through its loopholes.

But Allex still lived with the guilt, and said he’s never been able to fully move on from the memory of the day. The full audio story from StoryCorps is below.