24 powerful photos remembering 9/11 – twenty-one years on

It's been 21 years since the the 9/11 terrorist attacks that shook, and changed, the world.

At 8:46am on 11 September 2001, a plane flew into the North Tower of the World Trade Center in New York, killing the 76 passengers and 11 crew members on board instantly. While at first the media and onlookers believed what was happening to be an accident, it soon became clear this was a targeted terror attack. Just seventeen minutes after the first plane hit, the South Tower was also hit by a hijacked plane at 9:03am.

The Towers were quickly engulfed in flames, only made worse by the plane's jet fuel. Frantic workers tried to escape the burning buildings, while some made phone calls to loved ones to say goodbye. After burning for nearly an hour, the South Tower collapsed at 9:59am, followed by the North Tower at 10:28am.

As the horrific events unfolded at the World Trade Center, al-Qaeda extremists claimed responsibility for the attack. And, at 9:37am, a third plane crashed into the government's Pentagon building near Washington, killing all 64 passengers aboard and 125 in the building.

A further 44 passengers were killed on a fourth hijacked plane, although according to a report issued many years later, those onboard overpowered the terrorists and diverted the plane away from its intended target, thought to be the White House or the US Capitol. The plane crashed into a field in Pennsylvania at 10:03am.

The four attacks that took place on September 11 killed nearly 3000 people, and more than 2000 first responders who worked on the site have since died from cancers believed to be linked to 9/11. In the aftermath of the attacks, volunteers and rescue workers spent the next nine months sifting through the debris to search for survivors, recover bodies and clean up the wreckage.

Here, we look back at some of the most powerful photos from September 11 and the days that followed.