The four coordinated attacks on Sept. 11, 2001 killed nearly 3,000 people, including more than 340 firefighters and 72 law enforcement officers. Some 25,000 people were injured in the attacks, perpetrated by Islamist terrorist group al-Qaida.
Two decades later, Americans are assessing new threats, like cyberterrorism and domestic extremism, while reflecting on the tumultuous withdrawal of U.S. troops from Afghanistan.
Do you think 9/11 changed the world?
It made it clear that terrorism is a clear and present danger. The most deadly attack against America since WWII was not launched by a great power or any country, but by a small group of super-empowered terrorists.