Passenger jets, hijacked by al-Qaeda terrorists, were flown into the World Trade Centre towers, causing their collapse, while a third was flown into the Pentagon building in Washington DC. A fourth plane missed its target and crashed into a field in Pennsylvania thanks to the heroic efforts of the passengers and crew.
A number of documentaries and events have been planned to mark the sombre anniversary of the attacks, many of which will centre around the long-standing memorial at the former site of the Twin Towers - now referred to as Ground Zero.
Here’s what you need to know about the 9/11 memorial, and how the disaster will be remembered this year.
Where is the 9/11 memorial?
The Ground Zero memorial sits at the former site of the Twin Towers in Lower Manhattan, New York.
The September 11 museum, which charts the unfolding of the attacks and remembers those lost, is located on the same site.
When was the memorial built and who designed it?
The 9/11 memorial consists of two one-acre pools on the footprints of the former twin towers, containing the largest man-made waterfalls in the United States. Surrounding the pools is a field full of swamp white oak trees.
The names of the victims of the attacks are inscribed on parapets surrounding the waterfalls.
The memorial, titled “Reflecting Absence”, was designed by architect Michael Arad and landscape architect Peter Walker in 2004, having been selected from 5,201 entries in a design competition.
Construction was finally completed in 2011, coinciding with the 10th anniversary of the attacks.
What does the memorial mean?
With the pools sitting on the former site of the Twin Towers, they’re supposed to represent the physical void and loss of life created by the attacks.
The sound of the waterfalls is supposed to mute the sounds of the city, making the site a place for quiet contemplation and reflection. The trees are intended to add to this feeling.
The names surrounding the memorial are arranged in a very particular order, with the architects conducting interviews and taking requests to try and situate names next to each other according to whether the people were friends or had a connection in real life.
Names are also arranged around different pools or parts of pools depending on where the victims were during the attack.
What does the museum contain?
Opened to the public in 2014, the September 11 Museum has a collection of more than 40,000 images, 14,000 artefacts, more than 3,500 oral recordings, and over 500 hours of video related to the attack.
The museum is intended to commemorate the attack and its victims without creating further distress to family and friends of those who died.
How will the 20th anniversary be marked?
In New York, the National September 11 Memorial and Museum will lead commemoration of the victims on the 20th anniversary of the attacks.
At the site, the names of victims will be read aloud by family members, while “six moments of silence” will be observed to mark when each of the twin towers was struck and fell as well as the times of the attack on the Pentagon and the crash of Flight 93.
As the sun sets, the “Tribute in Light” will shine up into the sky from the site as it does on every anniversary of the attacks.
Around the world, vigils will be held in memory of the victims.