American Airlines Flight 77 from Washington Dulles to Los Angeles was one of four flights hijacked on September 11, 2001. The Boeing 757 was crashed into the Pentagon, killing 59 passengers and crew and 125 in the building as well.

About half an hour into the flight five hijackers stormed the cockpit, sent passengers and crew to the rear of the aircraft, and took control of the aircraft. Impact was at 9:37 a.m. Eastern time.

But two of the hijackers almost didn’t make the flight, and one American Airlines agent, Vaughn Allex, has lived with the guilt for 21 years that he let them check in when they were late arriving to the airport.

The Washington Dulles-based ticket agent saw two full fare first class passengers and wanted to offer his best customer service.

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 recently recalled in an interview at Dulles airport in Virginia.

“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.”

"I realize that there's probably nothing I could've done to prevent what happened. I've come to terms with that."

Vaughn Allex, former American Airlines gate agent, speaks about the guilt he feels for allowing hijackers onto Flight 77 on Sept. 11, 2001. https://t.co/qabkKxf6WD pic.twitter.com/HYj7VXTVRK

— ABC News Live (@ABCNewsLive) September 10, 2021

He marked the two men for extra security “[w]hen the pair couldn’t answer basic security check-in questions.” But they didn’t have bombs or guns, that’s not how they’d hijack the plane.

Allex also has guilt for suggesting a co-worker take the flight with a connection to Las Vegas, rather than going through Dallas or from Washington National to Chicago, since the transcon offered “”a meal and a movie” and was so lightly booked.

“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.”

The next day he was called in and introduced to an American Airlines lawyer who said, “I am not your attorney.” They were joined by two FBI agents.

“I started to run my hand down the list and I saw the names of the two people I checked-in, and in that moment and that instant, that’s when I looked at him and I said, ‘I did it, didn’t I?’ And they said, ‘what did you do?’ And I go, ‘these were the two that I put in,'”

Allex blames himself, though with the information he had he made the right calls all the way through. Playing his own role in that fateful day, being interviewed by the 9/11 Commission, it’s been hard to escape. Realizing that he was just listed as a footnote in the report (‘footnote number 12’) made him realize how many things happened that day, and how small a role he played. In 2008 he retired from American Airlines, and he went to work for TSA. Here’s an amazing year-old thread with an inside look at how the day unfolded.