As horrified viewers around the world watched the events of September 11, 2001 unfold, myriad conspiracy theories quickly spread across the internet in one of the earliest forms of modern misinformation.
These falsehoods, supported by the 9/11 "Truth" movement, reject the consensus that Al Qaeda were responsible for the attacks. Instead, they claim the US government was involved, that a missile struck the Pentagon and that explosives were planted at the World Trade Centre, among many other theories.
Conspiracy theories surrounding the attacks have since been widely debunked, but nevertheless persist, continuing to circulate online 20 years after the attacks.
Explosives planted at North and South Towers
The phrase “jet fuel can't melt steel beams” became a leading argument among “truthers” - 9/11 conspiracy theorists - after the September 11 attacks.
Conspiracy theorists claim the two hijacked planes - American Airlines Flight 11 and United Airlines Flight 175 - that flew into the North and South Towers could not have caused the buildings to collapse, and that explosives must have been planted.
A years-long investigation by the National Institute of Standards and Technology (NIST) found this claim to be untrue.
Though the steel did not melt, the institute estimated it would have lost about 90 per cent of its integrity from the fires the erupted when the planes hit the building.
The NIST investigation outlined why the North and South Towers collapsed. First, the impact of the planes critically damaged the buildings' support columns, which dislodged the fireproof insulation. This caused the jet fuel to spread across several floors, allowing the fires to spread unchecked.
Second, the columns and floors within the buildings were significantly weakened by the fires. This, the institute concluded, led to the inward bowing of the perimeter columns and the buildings' eventual collapse.
WTC 7 destroyed by controlled demolition
For truthers, World Trade Centre 7 is clear evidence that 9/11 was an “inside job".
The building housed the offices of both the secret service and the CIA, and conspiracy theorists claim that the 47-storey building was destroyed in a controlled demolition to wipe out evidence that the US government was involved in the attacks.
WTC 7 was not hit by a plane and showed no exterior damage following the attacks.
Footage of the collapse itself, which appeared to show World Trade Centre 7 descend at a free-fall speed, was given as evidence for the building's destruction by a controlled demolition.
But while there was no obvious damage to the exterior of the building, the NIST investigation found that WTC 7 was strongly affected by the aftereffects of the attacks.
Debris from the fallen North Tower sparked uncontrollable fires inside World Trade Centre 7 that lasted for hours.
The NIST found that the heat led to a chain of events that weakened the building's structural integrity, with its middle severely compromised. A rapid succession of structural failures caused the east and west sides to collapse on themselves before the building caved in.
A missile struck the Pentagon
Truthers also claim that the hole in the Pentagon building - just shy of five metres across - was too small to have been made by a Boeing 757, and conspiracy theorists point to this as evidence that the building was struck by a missile, not a plane.
But that claim ignores video evidence of the attack.
Security camera footage released in 2004 showed that American Airlines Flight 77 struck the west side of the Pentagon.
The size of the entry hole was also misreported. The official ASCE Pentagon Building Performance Report estimated that the hole was closer to 23 metres. The destruction of the plane's wings before impact accounted for the hole not being larger.
Another claim states that parts of a scrapped plane were transported to the Pentagon to give the impression that it had been hit by an aircraft. But there were dozens of eyewitnesses who saw the plane hit the building and the Pentagon received 24-hour news coverage directly following the attacks, making it impossible for anyone to plant anything.
The attack on the Pentagon killed 125 people in the building and the 59 passengers and crew on board the plane.