"I blamed myself," says Vaughn Allex — who also convinced a close friend to board the same plane.
Twenty years later, Vaughn Allex will never forget the moment he assisted two late passengers make it aboard their flight at the last minute on the morning of September 11, 2001.
In a poignant interview with ABC, the former American Airlines ticket agent recalled rushing to help Brothers Salem and Nawaf Al-Hazmi make AA Flight 77 — a flight they would hijack minutes later and crash into the Pentagon, killing 189.
Allex remembers the two well, saying they were acting "odd".
"I realize that there's probably nothing I could've done to prevent what happened. I've come to terms with that."
"They're late, they stopped, they were confused, they were looking back and forth — and then they made a beeline for where I was working with my agents," he said, while standing at the exact same counter the encounter took place at Dulles International Airport. "I was joking with my agents, I said 'Watch: these two are for Flight 77.' And they were."
Allex was training two new employees at the time. With less than 20 minutes to departure, he insisted on showing them how airline policy was to do everything they could to help late first class passengers make their flight.
"I told the agents, because they were new: 'Let me show you how we handle late arriving passengers," he recalled. "And we did everything according to the book."
It was a decision that would weigh on him for the rest of his life.
"The check-in was odd. The two that I checked in, two brothers, one was kind of gruff and the other one was standing a couple of paces behind him. And this sounds odd, but this is what caught my attention. He was almost dancing, he was moving from foot to foot and grinning and looking around, and my thought was, here's somebody that's never been on an airplane and boy is this guy excited," Allex explained.
"And I kind of watched him for a couple of minutes as we went through the whole check. And he was totally unresponsive as far as whatever we asked him to read, to look verbally. He just smiled and danced and was oblivious to what was going on," he continued. "That's the image I have, is the two of them standing there and the one just dancing, it was the oddest thing."
When they couldn't even answer basic security check-in questions, he marked their tickets for additional security. One of the brothers even set off the metal detector. But they made the flight nonetheless.
He also remembers the very moment he learned what had happened.
"I walked into the back room to talk to the supervisor on duty, and I said 'There's a rumor about 77, tell me that it's not true,'" he recalled. "And the three of them just turned around and looked at me, and it was like: 'Get out.'"
"I knew all of the cabin crew. I worked with them for years," he said. "What I didn't know until about mid-morning, when the FBI was talking to me, was that those last two passengers that I checked in were actually two of the hijackers. I looked at them and said 'I did it, didn't I?'"
"I put those two on the flight. And it was just devastating. It was just absolutely devastating," he said. "I had no idea until that moment that I had been involved in it."
For years to come, Allex would be wracked with guilt.
"I blamed myself," he told ABC. "I thought, you know, if I had done something different, if I had not let them on, if I had just said to the agents 'These two guys are let, let them get the next flight, we have one at noon, it's no big deal.'"
Allex is also tortured by the fact that one of the victims was his close friend and colleague MJ Booth — whom he had convinced to get on the flight. Trying to get to Las Vegas, she had been considering flying via Chicago or Dallas, but Allex advised her to go though LA instead, on Flight 77.
"I said, first of all, it's a better flight. It's a transcontinental flight. You get a meal and a movie and it's relaxing," he recalled. "She said that sounded good, but that she'd never written a ticket that way and we were just transitioning to electronic tickets. Could I help her? So I wrote her ticket from Dulles to Los Angeles with a connecting flight back to Las Vegas. And then the following day, I saw that she had gotten on the flight on the ticket I'd written."
Despite being told for years by friends and colleagues that it wasn't his fault, that he was just doing his job, he said it wasn't until 2004, when he bought a copy of the 9/11 Commission Report — for which he was interviewed — that he finally began to forgive himself.
".Here is this book with hundreds and hundreds and hundreds of pages, and I'm on page three. I have a little paragraph and a footnote, footnote number 12," he said.
"That's when it started to get better," he added. "That's when I went — 'Oh my gosh. There were so many other people involved, there were so many innocent people that just touched on this. And I had just such a small, tiny five-minute part of it.' But before that, it was -- it was terrible."
After leaving his job at American Airlines, Allex went on to work for the TSA, hoping to help keep future fliers safe.