While the September 11 attacks are still fresh in the memory of many of us who watched the collapse of the Twin Towers on TV, this year marks 20 years since the disaster that claimed the lives of nearly 3,000 people in the United States.
But when exactly did the terrorist attack take place, how many people were killed, and who carried it out?
This is what you need to know.
What year was 9/11 and when did it take place?
On 11 September 2001 four coordinated terrorist attacks were carried out by al-Qaeda members in the United States.
The Twin Towers were the tallest buildings in New York City. At 110 stories each, 1 WTC (North Tower) and 2 WTC (South Tower) provided nearly 10 million square feet of office space for about 35,000 people and 430 companies.
At 8:46 am (Eastern Time), hijacked Flight 11 crashed into floors 93 to 99 of the North Tower, 1 WTC.
The hijacked Flight 175 struck floors 77 to 85 of the South Tower (2 WTC) 17 minutes later at 9:03 a.m.
When the towers were struck, between 16,400 and 18,000 people were in the WTC complex.
The fires from the impacts were intensified by the planes’ burning jet fuel. They weakened the steel support trusses, which attached each of the floors to the buildings’ exterior walls.
Along with the initial damage to the buildings’ structural columns, this ultimately caused both towers to collapse. The five other buildings in the WTC complex were also destroyed because of damage sustained when the Twin Towers fell.
A third plane which was hijacked over Ohio, crashed into the Pentagon building in Virginia at 9.37am, causing a partial collapse of the building’s west side.
The fourth and final flight, United 93 - most likely headed for the Capitol building or the White House in Washington DC - missed its intended target and crashed into a field in Pennsylvania at 10.03am.
How many people died?
During the September 11 attacks, 2,977 people were killed in total, and more than 6,000 people were injured.
The deaths included 265 people on the four planes that were hijacked - including the terrorists - as well as 2,606 people in the World Trade Center and surrounding area.
Most of the people who died were civilians, however 344 firefighters, 71 law enforcement officers and 55 military personnel were also killed.
A total of 125 people were killed at the Pentagon. The attacks were the deadliest terrorist act in world history.
Who carried out the attacks?
The US responded to the 9/11 attacks by launching a War on Terror and invading Afghanistan to depose the Taliban, which had not complied with its demands to expel Al-Qaeda from the country and extradite its leader Osama Bin Laden.
The Taliban housed al-Qaeda in Afghanistan in the late 1990s and early 2000s.
Al-Qaeda operated training camps there, and openly lived in the country with the support of the Taliban, which ruled Afghanistan at the time.
On September 20, 2001, in a speech to a joint session of Congress, former President George W. Bush asserted: “Any nation that continues to harbor or support terrorism will be regarded by the United States as a hostile regime.”
No distinction was made between a harboring state and the terrorists it was harboring.
The U.S. government insisted that the Taliban immediately hand over the terrorists and close the training camps or face an attack from the United States.
When they refused, “Operation Enduring Freedom” was launched on October 7, 2001, less than a month after the attacks of 9/11.