The man who checked in the hijackers who crashed Flight 77 into the Pentagon on 9/11 said it's haunted him ever since.
On 11 September, 2001, a total of four passenger planes were hijacked with the intention of crashing them into important US buildings in a terrorist attack that shocked the world and killed almost 3,000 people.
Everyone remembers the two planes that hit the World Trade Centre in New York City, destroying the Twin Towers, but there were two other planes hijackers also took over on 9/11.
American Airlines Flight 77 was hijacked and crashed into the Pentagon, the home of the US Department of Defense, killing 189 people.
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A fourth plane, United Airlines Flight 93, was headed for Washington DC and was supposed to be crashed into the US Capitol Building.
However, passengers attempting to retake control of the plane were able to bring it down in a field, killing all 44 people on board but potentially sparing many more lives through their actions.
Vaughn Allex was working for American Airlines at the ticket counter of Dulles International Airport, Virginia, on the day that shook the world, and has shared the story of how checking two of the hijackers onto Flight 77, the plane that was crashed into the Pentagon, has haunted him ever since.
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He spoke to Story Corps about that day, remembering the two men who were running late to board their flight and who came rushing up to his check-in desk.
Those two men were Salem and Nawaf Al-Hazmi, brothers who would be part of the five man team that hijacked the American Airlines flight and crashed it into the Pentagon.
Checking them onto the flight, Allex didn't fully realise what had happened or what small part he had unknowingly played until the next day when he went into work and 'people wouldn't look at me in the eye'.
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Allex remembers many of the other people he checked onto Flight 77 that day, and spoke about how difficult it was to come to terms with 9/11.
He said: "I checked in a family that was a retiree and his wife, I had time to talk to them.
"There was a student group and I checked in a lot of those kids, and the parents, teachers. They were gone, they were just all gone. Once it became known people didn't talk to me."
"I felt that there was no place for me in the world, there were all these support groups and I didn't belong there because how do I sit in a room with people that are mourning and crying and they're like 'what's your role in this whole thing' well I checked in a couple of the hijackers and made sure they got on the flight."
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He explained that while it's something that has haunted him ever since and he won't ever forget, he has in recent years been more able to talk about it.
In truth there was no way he could have known what was about to happen, on that day all he saw were a couple of passengers with tickets rushing to get onto their flight.
More than 20 years after the 9/11 terrorist attacks, efforts to provide pictures for all of the victims who lost their lives are still ongoing.
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The recent death of Ayman al-Zawahiri, referred to as Osama Bin Laden's right hand man, in a drone strike means that all 9/11 plotters have been either killed or apprehended.
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Joe Harker