9/11 conspiracy theories – what are the common conspiracies surrounding the September 11 attacks and how have they been debunked?

TODAY marks 18 years since the 9/11 terror attack wiped out the iconic Twin Towers in New York and killed 2,996 people across four sites.

Despite the attack being broadcast live across global TV screens, there are still conspiracy theories surrounding the horrific events of September 11, 2001. We look at the most common ones still floating around.

When was 9/11?

On September 11, 2001, a group of Al-Qaeda terrorists hijacked four commercial airliners.

Two planes - American Airlines Flight 11 and United Airlines Flight 175 - were flown into the North and South towers of the World Trade Center complex in New York.

Another was flown into the Pentagon in Washington and the fourth, United Airlines Flight 93, crashed into a field in Pittsburgh, Pennsylvania, after a struggle between the hijackers and passengers.

How many conspiracy theories are there?

There are dozens of conspiracy theories floating around 18 years after the attacks took place.

The most well-known conspiracy theory relates to the collapse of World Trade Center Tower 7.

It fell later after the collapse of the neighbouring Twin Towers.

The conspiracy theory suggests the collapse of the buildings was not solely caused by the plane attacks but also by explosives already installed in the towers.

The official report into the tragedy cites flaming debris from the burning skyscraper crashing into the 47-floor Tower 7, sparking fires on several floors.

The heat generated brought the tower down, concluded the National Institute of Standards and Technology – making it the first steel skyscraper in the world to collapse because of fire.

In November 2016, a group of top engineers from the University of Alaska said “office fires” could not have caused its destruction.

Presenting the team’s findings at the Justice In Focus Symposium in New York, leader Dr J Leroy Hulsey revealed: “It is our preliminary conclusions based upon our work to date that fire did not produce the failure at this particular building.”

Their verdict stoked theories that the World Trade Center buildings were brought to the ground by controlled demolition explosions but there is no evidence to support such a theory.

What are the other most circulated theories?

Failure to intercept the hijacked planes:

Conspiracy theorists have always questioned why the US Air Force did not intercept any of the four hijacked planes.

They claim that then-US Vice President Dick Cheney ordered the military to stand down and not to intercept.

However, official reports claim that this was a highly unusual multiple hijacking with on-board violence.

The transponders of all four aircraft were also turned off or changed, meaning the planes could not be identified.

The hijacking also took place on a day when routine military training exercises happened to be taking place at US air defence command.

Data suggests there was confusion and a lack of communication between the civilian air traffic control (FAA) and the military at the time.

Attack on the Pentagon:

Theorists have repeatedly questioned how an amateur pilot was able to fly a commercial plane in a complicated manoeuvre and crash it into the headquarters of the world's most powerful military, 78 minutes after the first report of a possible hijack, without leaving any traces.

Based on this question, they claim that instead of a Boeing 757, the Pentagon was in fact struck by a small missile or unmanned drone, or even a significantly smaller aircraft.

When it was confirmed that flight AA77 did hit the Pentagon, the theorists shifted their focus to how the plane managed the difficult manoeuvre, with hardliners claiming the plan was actually being controlled by the Pentagon itself rather than Al Qaeda terrorists.

While some early video footage did not show much wreckage at the Pentagon, there is a good deal of video and still photography from the aftermath which shows plane wreckage and evidence of the flight path, such as broken lamp posts.

The remains of crew and passengers on the plane were also found and positively identified by DNA, and multiple witnesses also saw the plane strike the Pentagon.

The fourth plane – United Airlines flight 93 crash site

Conspiracy theorists chose to focus their questions on why the crash site at Shanksville, Pennsylvania, was so small and why aircraft debris was not as visible as other sites.

They claim instead that the plane was shot down by missiles, causing it to disintegrate mid-air – this would have scattered the wreckage over a large area.

However, this has been refuted by official photographs that show the full wreckage and recordings of the pilot’s voice confirmed that there had been a passenger revolt when hijackers had attempted to take over the aircraft.

The pilot can also be heard confirming that the hijackers caused the plane to crash.

The 'planes' were holograms:

Among the most absurd claims made by conspiracy theorists include saying the planes were not real.

They suggest that the “planes” that hit the Twin Towers and the Pentagon were missiles surrounded by holograms that were made to appear as planes.

However, while this is a popular theory, it fails to take into account the missing planes, passengers and hijackers.

It was an insurance scam

One theory revolves around New York property tycoon Larry Silverstein who purchased the entire complex on a nine-year lease six months prior to the attacks.

The deal was worth $124million (£96million) - but he is said to have walked away with $4.5billon (£3.2billion) after taking out a huge insurance claim following the tragedy.

Not only that, Mr Silverstein also reportedly explicitly included "terrorist attacks" as a clause in the new agreement - which fact-fearing oddballs see as a sign of an inside job.