WARNING: Distressing content

On this day 18 years ago, unimaginable horror struck America when four commercial flights were hijacked by terrorists and crashed into sites including New York City's Twin Towers.

The morning of Tuesday, September 11 was fine and sunny, with New Yorkers navigating the manic rush-hour commute to work or school under a cloudless blue sky.

But at 8.46am the unthinkable happened: American Airlines Flight 11, bound for Los Angeles, was hijacked by members of Al-Qaeda at Boston airport and flown into the North Tower of the World Trade Centre.

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People stared up in confusion and disbelief at the smoking building, wondering if perhaps it was an accident, until 17 minutes later a second plane, United Airlines Flight 175, hit the South Tower at 9.03am.

By 10.30am the Twin Towers had collapsed, sending people running for their lives covered in dust and debris. Meanwhile, 370 kilometres away, American Airlines Flight 77 was flown into the Pentagon building in Virginia at 9.37am, and United Airlines Flight 93 was crashed into a field near Shanksville, Pennsylvania, at 10.03am.

In less than two hours, 2996 people had been killed and more than 6000 injured, including hundreds of firefighters and police officers who rushed to the scenes.

Photographers captured the event in unforgettable images.

The hijacked plane approaching the World Trade Centre. Photo / AP
The hijacked plane approaching the World Trade Centre. Photo / AP
Smoke rises from the burning twin towers of the World Trade Centre on September 11, 2001. Photo / AP
Smoke rises from the burning twin towers of the World Trade Centre on September 11, 2001. Photo / AP
The fiery blasts after two planes hit the twin towers of the World Trade Centre on September 11, 2001 in New York City. Photo / Getty Images
The fiery blasts after two planes hit the twin towers of the World Trade Centre on September 11, 2001 in New York City. Photo / Getty Images
Hijacked United Airlines flight 175 flies towards the south tower of the World Trade Centre on September 11, 2001 in New York City. Photo / Getty Images
Hijacked United Airlines flight 175 flies towards the south tower of the World Trade Centre on September 11, 2001 in New York City. Photo / Getty Images
A firefighter covered with ash is helped to safety after the collapse of the World Trade Centre. Photo / Getty Images
A firefighter covered with ash is helped to safety after the collapse of the World Trade Centre. Photo / Getty Images
Firefighters work at the destroyed World Trade Centre after the September 11, 2001 terror attack. Photo / AP
Firefighters work at the destroyed World Trade Centre after the September 11, 2001 terror attack. Photo / AP
Secret Service agent Thomas Armas, carries an injured woman to an ambulance after Tower One of World Trade Centre collapsed. Photo / Getty Images
Secret Service agent Thomas Armas, carries an injured woman to an ambulance after Tower One of World Trade Centre collapsed. Photo / Getty Images
A photo lies in the rubble of the collapsed twin towers. Photo / AP
A photo lies in the rubble of the collapsed twin towers. Photo / AP
The Pentagon damaged by a hijacked plane on September 11, 2001. Photo / AP
The Pentagon damaged by a hijacked plane on September 11, 2001. Photo / AP
Former chief of staff Andy Card whispers into the ear of then president George W. Bush to tell him of the plane crashes into the World Trade Centre during a visit to a Florida school. Photo / AP
Former chief of staff Andy Card whispers into the ear of then president George W. Bush to tell him of the plane crashes into the World Trade Centre during a visit to a Florida school. Photo / AP
This photo became known as 'The Falling Man' - the unknown man falls from the North Tower of the World Trade Centre at 9.41:15am on the morning of the terrorist attacks. Photo / AP
This photo became known as 'The Falling Man' - the unknown man falls from the North Tower of the World Trade Centre at 9.41:15am on the morning of the terrorist attacks. Photo / AP
Firefighters search for survivors in the rubble. Photo / AP
Firefighters search for survivors in the rubble. Photo / AP
People hang from the windows of the north tower of the World Trade Center after a hijacked airliner hit the building on September 11, 2001. Photo / Getty Images
People hang from the windows of the north tower of the World Trade Center after a hijacked airliner hit the building on September 11, 2001. Photo / Getty Images
People run from a cloud of debris from the collapse of a World Trade Centre tower in New York. Photo / AP
People run from a cloud of debris from the collapse of a World Trade Centre tower in New York. Photo / AP
A firefighter breaks down after the World Trade Centre buildings collapsed September 11, 2001 after two hijacked airplanes slammed into the twin towers in a terrorist attack. Photo / AP
A firefighter breaks down after the World Trade Centre buildings collapsed September 11, 2001 after two hijacked airplanes slammed into the twin towers in a terrorist attack. Photo / AP