Almost 3,000 people were killed in the 9/11 attacks after al-Qaeda terrorists hijacked four commercial airliners; two passenger jets were flown into the World Trade Centre towers, causing their collapse.
A third plane hit the Pentagon just outside Washington DC, and the fourth and final plane crashed into a field in Pennsylvania.
But many 9/11 conspiracy theories have appeared which say that certain people in the United States government knew about the attacks, or even made them happen.
And some - despite countless hours of historical footage on the contrary - say the Twin Towers may never have fallen at all.
Here is everything you need to know about them.
What do conspiracy theorists believe?
There are many conspiracy theories that attribute the planning and execution of the 11 September attacks against the United States to parties other than al-Qaeda.
Some theories suggest the attacks were carried out by the terrorist organisation, and high-level government officials had advance knowledge of their plans but chose to ignore them, or even assist the attackers.
The most prominent conspiracy theory is that the collapse of the Twin Towers was the result of controlled demolitions, rather than structural failure due to impact and fire.
Government investigations and independent reviews have rejected these theories, including the National Institute of Standards and Technology (NIST), which found conclusive evidence that the impacts of jet aircraft at high speeds in combination with subsequent fires led to the buildings’ collapse.
But still theorists - such is their nature - are not satisfied by the facts.
They claim that the impact of a fully loaded passenger plane, which would add tremendous tonnage to the buildings once its wreckage was lodged in, would not cause enough damage to topple the buildings.
Maybe not on its own. But the fires that ravaged through the building in the aftermath would have been able to reach extremely high temperatures, fuelled by burning jet fuel in contained conditions that would only have added to their temperature.
Molten metal created by the fires would then have trickled down to lower floors, sparking more fires and causing structural damage everywhere it went.
Eventually, the weakened building could no longer support its own weight, and both of the Twin Towers - as well as World Trade Centre 7 - collapsed.
Since 9/11, at least two steel-framed high-rise buildings have collapsed solely due to fires — the Plasco Building in Tehran in January 2017, and the Wilton Paes de Almeida Building in São Paulo in May 2018.
Why do conspiracy theorists believe it?
Possible motives claimed by conspiracy theorists for such actions include justifying the invasions of Afghanistan and Iraq, strengthening America’s political interests in the Middle East.
Theorists claim that America and the UK both had desires on invading the countries before 9/11, and that the attacks served as a perfect reason for doing so.
They cite plans to construct a natural gas pipeline through Afghanistan as a major reason for America’s apparent want to control the country.
They assert that the attacks were either an “inside job”, using controlled demolition techniques to bring the towers down, or that American authorities knew about them, and chose to let them play out to their advantage.
All of these theories have since been proven to be false.
The no plane theory
One of the most “out there” theories dares to suggest there were no planes involved in 9/11, and no hijackers - a theory that is so wild, discussion of it has been banned from many conspiracy websites.
People who believe this theory argue that it would have been physically impossible that the Boeing planes could have penetrated the steel frames of the Towers.
But the event was documented by countless news crews and amateur videographers, with every angle clearly showing the second plane hit the towers.
With fewer eyes trained on the buildings at the time of the first aircraft strike, footage of the first plane’s impact is harder to come by, but it does exist.
So what explains the clear footage of aircraft crashing into the buildings? CGI of course.
Believers say digital compositing was used to depict the plane crashes in both news reports and amateur video, and that CGI of a passenger plane was overlaid onto a winged cruise missile or military aircraft.
According to British former MI5 officer and 9/11 conspiracy theorist David Shayler, "the only explanation is that they were missiles surrounded by holograms made to look like planes.”
"Watch footage frame by frame and you will see a cigar-shaped missile hitting the World Trade Centre."
However, hologram technology has not reached that level of complexity today, let alone 20 years ago.
A message from the editor:
Thank you for reading. NationalWorld is a new national news brand, produced by a team of journalists, editors, video producers and designers who live and work across the UK. Find out more about who’s who in the team, and our editorial values. We want to start a community among our readers, so please follow us on Facebook, Twitter and Instagram, and keep the conversation going. You can also sign up to our newsletters and get a curated selection of our best reads to your inbox every day.