
Warning: This article contains details which some readers may find distressing.
On 11 September 2001, the course of US history changed forever when al-Qaeda terrorists hijacked four commercial airlines.
Three of the planes hit their intended target - New York's Twin Towers and the Pentagon - while a fourth, headed towards the White House, crashed in a Pennsylvania field after a passenger revolt.
A quarter of a century on from the attacks, National Geographic has created a new three-part documentary series - titled 9/11 Reunited - which tells the story of those who witnessed the atrocity first-hand and reunites people whose lives were saved by strangers.
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This includes Stefan Horlacher, a man who witnessed the dark aftermath of American Airlines Flight 11 colliding with the North Tower.
A bond trader originally from Germany, Stefan was working two blocks away from the World Trade Centre complex that morning and remembers feeling a 'vibration' as the aircraft collided with the first tower.
Curious about what had just happened and wanting to see if he could help, Stefan left his office and walked over to a bookstore on the complex, where he witnessed frightened workers leaping to their deaths in order to escape the burning skyscraper.
"We had the first jumpers, splashing on the plaza," Stefan explains in an interview with LADbible.
"Almost every 10 seconds, maybe 20 seconds, somebody right in front of you, kind of splashed on the plaza.
"They always gave [me] that deep, dark noise... Then you knew it was more serious than just a tiny accident."


"You're helpless in that situation," he added in the documentary. "You couldn't do anything at all."
As many as 200 people jumped to their deaths during the 9/11 attacks
Some of the most harrowing photographs taken on 11 September 2001 were images of people falling through the air after jumping from higher stories of the building
It's estimated that roughly 200 people fell to their deaths from the Twin Towers, the majority of whom were believed to have jumped deliberately to escape the flames which trapped them in the higher levels of the building.
Stefan wasn't the only person who witnessed people jumping to their deaths, with retired firefighter Bill Spade recalling first responders having no choice but to drive over human remains as they approached the towers.
"I said a prayer," Spade, the only surviving member of FDNY's Rescue 5 who now gives public speeches about his experience, previously told the New York Post. "Body parts and corpses filled the street... There was no way to avoid running over some of them."
The falling civilians would also cause further fatalities on the ground, with firefighter Daniel Suhr becoming the first FDNY fatality of the day after he was killed instantly by a person who'd jumped from the North Tower.

"It was like this huge fireball… all these pieces coming down."
After realising there was nothing he could do to help, Stefan began walking away from the plaza, which is where he saw the second plane collide with the South Tower.
"This huge fireball is just coming out and coming down with all the debris," he remembered of the moment.
Unknown to him at this point, Stefan was standing next to a woman named Debbie St John.
The 30-year-old had been on her way to work that morning when she too was caught up in the aftermath.
A falling piece of debris hit Debbie, likely the blade of one of the aircraft's engines, cutting through her back and causing extensive injuries to the lower half of her body.
“Debbie was lying on the street and very afraid," Stefan tells LADbible. "I knew I had to help her.
"I noticed that she had some problems in her feet and there was some blood, but she didn't move. I took off my shirt because I wanted to protect her, cover her up"

The bond trader and a few other witnesses then worked together to protect Debbie from the ongoing stampede of terrified civilians trying to escape the chaos. He wrapped his shirt around Debbie's torso, careful not to cause further trauma to her extensive injuries, and waited with her until emergency services arrived.
"You don't forget that," he adds.
The first hospital patient admitted on 9/11 and the last to leave, Debbie would spend a total of 18 months in hospital and undergo a total of 50 surgeries to save her life.
Although the injuries would have a lasting impact on her life, Debbie was able to rebuild her life, marrying her partner at the time and going on to have two children.

She was able to come face-to-face with Stefan for the first time since the accident during the documentary, allowing Debbie the opportunity to thank Stefan for risking his life to save her that day.
"I lost a lot of friends in the attack," Stefan added. "But that [Debbie] survived, gave me a positive attitude. Even though I was helpless, I helped someone."
9/11 REUNITED streams on Disney+ from 22 August and premieres on National Geographic on Sunday 23 August from 9pm, with the series repeated on the channel on Friday 11 September from 8pm.
Topics: TV, Documentaries, US News, New York