This year marks the 20th anniversary of the 9/11 attacks, which saw terrorists take control of planes over the US and turn them into deadly weapons.
Shortly before 9am, New York was turned into a scene of devastation after two planes smashed into the World Trade Center buildings.
In the 20 years since the attacks, the skyline of New York has changed, wars have been waged and Khalid Sheikh Mohammed – the principal architect behind the plot – has been captured and detained.
Terrorists also attacked the Pentagon in Washington DC, while those on board United 93 were thought to have been targeting either the White House or the Capitol before passengers decided to act and the plane came down before it reached the US capital.
But it was the images from the World Trade Center that are the most closely associated with a day that will never be forgotten.
When did 9/11 happen?
On Tuesday, 11 September 2001, 19 al-Qaeda terrorists boarded several planes with a mission to kill as many people as possible.
American Airlines Flight 11 was the first plane to reach its target, hitting the North Tower of the World Trade Center at 8.46am.
There were mixed reports as it was at first believed by some that a small private plane had hit the tower by accident but less than 20 minutes later, it was obvious that a terror attack was taking place as United Airlines Flight 175 smashed into the South Tower of the twin towers at 9.03am.
Watch: 9/11 terror attacks 20 years on
Away from New York, American Airlines Flight 77 crashed into the Pentagon at 9.37am, and authorities were alarmed about United Airlines Flight 93, which was not responding to air traffic control and had veered off course.
Orders had been given to shoot the plane down if necessary, with concerns that it was aiming to crash into the White House or the Capitol in Washington DC.
But the plane crashed on the ground near Shanksville, Pennsylvania, at 10.03am after passengers and crew attempted to seize control from hijackers before the terrorists rolled the plane over and flew it into the ground.
How many people died?
The 9/11 attacks remain the deadliest terrorist attack in history, resulting in a total of 2,977 deaths, while all 19 hijackers were also killed.
The vast majority of deaths occurred in New York, with 2,606 people killed either inside the World Trade Center or on the ground below.
At the Pentagon, 125 people were killed while all 39 civilians on board United 93 were killed when it crashed into a field.
Over 90 countries lost citizens in all the attacks, including 67 from the UK – the most non-US deaths in the 9/11 attacks.
In the aftermath of 9/11, the World Trade Center site became known as Ground Zero, which stretched over an area of 14.6 acres.
The twin towers were one of the most recognisable building of the Manhattan skyline and the outline radically changed after the buildings fell.
Hundreds of thousands of tonnes of debris had to be cleared following the attacks, while it took years to identify victims from body parts discovered at the site.
Plans to rebuild the World Trade Center were put in place soon after 9/11 and 20 years later, several new structures have been put in place – the most visible being One World Trade Center, a 1,776 feet (541 metres) skyscraper that is the tallest building in the US.
Osama Bin Laden
While Khalid Sheikh Mohammed is regarded as the principal architect of the 9/11 attacks, it is al-Qaeda leader Osama Bin Laden who is most widely identified with the plot.
After becoming president in 1992, Bill Clinton made capturing Bin Laden a priority for the US following a series of attacks on US citizens at home and abroad.
However, it was 9/11 that ramped up the search to find him and a $25m reward was offered for information that led to the capture or killing of the terrorist leader.
Bin Laden’s whereabouts were a subject of confusion, with reports of him hiding out in mountains in caves in Afghanistan proving not to be fruitful, while it was also claimed he was attempting to avoid capture in neighbouring Pakistan.
For years the searches turned up nothing but US intelligence agencies finally tracked him down to a secure compound in Abbottabad, Pakistan, in 2011.
The house was raided by SEAL Team Six on 2 May, 2011, and Bin Laden was killed inside – nearly 10 years after the 9/11 attacks took place.
Watch: 9/11 survivor recounts remarkable escape from World Trade Center