The 9/11 attacks in New York and Washington DC shocked the world, with the ramifications still being felt over 20 years later.

Most people who were alive at the time can remember exactly what they were doing as the events unfolded.

Millions looked on in horror as four airliners were hijacked by 19 Al Qaeda terrorists and flown towards major sites in the US.

The atrocity was the single deadliest peace time attack on American soil and killed nearly 3,000 people.

On the 21st anniversary of the attacks on the Twin Towers, Metro.co.uk takes a look at how the events unfolded.

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Timeline of 9/11 terror attacks on Twin Towers

The famous New York towers were targeted for the first time by terrorists. Six died after a truck bomb exploded below the north tower, but many more lives are saved after the towers failed to collapse.

Pakistani terrorist Ramzi Yousef detonated a bomb on board a plane bound for Tokyo, killing one. A bigger plot to bomb 11 airliners and crash a plane into the CIA headquarters was foiled but it sowed the seeds for 9/11.

Saudi Arabians Nawaf al-Hazmi and Khalid al-Mihdhar, two members of the so-called ‘Hamburg Cell’ that was personally selected by Bin Laden, arrive in the US. They are joined in the coming months by fellow co-conspirators.

Egyptian terrorist Mohamed Atta, later identified as the ringleader of the attacks, emails flight schools enquiring about ‘intensive training programmes’. He later received a pilot’s licence.

Atta called planner Ramzi Binalshibh and said he had a riddle that he was trying to solve: ‘Two sticks, a dash and a cake with a stick down—what is it?’ Binalshibah realised the two sticks stood for number 11 and the cake with a stick down was a nine, meaning the attack was set for 9/11.

The first hijackers pass through security in Portland, Maine, and board a flight to Boston, where they connect to American Airlines Flight 11 – the plane that would hit the north tower.

Flight 175, which would later hit the south tower, takes off from Boston for Los Angeles. Meanwhile, American Airlines Flight 77, to Los Angeles, takes off from Washington, D.C. This flight later hits the Pentagon.

Ground personnel receive the first signal something is wrong when flight 11 crew members issue a hijacking alert. A passenger, Daniel Lewin, is stabbed by a hijacker who was seated behind him. He may have been trying to stop the attack from taking place.

This flight from Newark, bound for San Francisco, had seven crew members, 33 passengers, and four hijackers on board. It is claimed the terrorists intended to fly it into the US Capitol building.

Flight 11 crashes into floors 93 through 99 of the North Tower. President George Bush is alerted four minutes later and his advisors initial assumed it was a tragic accident.

As officials scramble to evacuate the World Trade Centre complex, Flight 175 crashes into floors 77 through 85 of the South Tower.

American Airlines Flight 77 crashes into the Pentagon in Washington DC. All 59 passengers on the plane are killed along with 125 on the ground.

As the world watches on, the south tower of the World Trade Centre collapses in 10 seconds after burning for 56 minutes. More than 800 people in and around the building are killed.

Crew and passengers storm the cockpit and bring the plane down near Shanksville, Pennsylvania – preventing further loss of life.

People are seen jumping from the upper levels of the building before the second tower collapses after burning for 102 minutes. More than 1,600 in and around the building are killed.

President Bush, who had travelled back to the White House from Florida, assured Americans that a search is underway for ‘those who are behind these evil acts.’

Some 20 people who were in the towers when they collapsed were saved. Rescue workers pulled Genelle Guzman-McMillan from the rubble 27 hours after the north tower fell. She was the last person found alive.

Bush announced that the attacks had been organised by Bin Laden and Al-Qaeda and declared a ‘war on terror’ while speaking to the United States Congress

The US and its allies – including the UK – launched Operation Enduring Freedom in Afghanistan. The objective was to remove the Taliban from power.

Despite widespread opposition at home and abroad, Bush and UK Prime Minister Tony Blair began the invasion of Iraq to remove Saddam Hussein from power.

Almost a decade after the attacks, Bin Laden is captured and killed by US forces in Pakistan.

To mark a decade since the devastating attacks, a memorial is opened in New York on the site of the World Trade Centre. It includes two large fountains on the sites where the towers stood.

Days before the 20th anniversary, two new victims were officially identified thanks to new DNA sequencing technology.

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