Updated 2154 GMT (0554 HKT) September 10, 2019
In an attack orchestrated by al Qaeda leader Osama bin Laden, 19 men hijacked four passenger planes on September 11, 2001.
Two of the planes were intentionally crashed into the World Trade Center towers in New York. Another crashed into the Pentagon. The fourth crashed in a field near Shanksville, Pennsylvania. Nearly 3,000 people were killed.
These photos show how the day unfolded.
Editor’s note: This gallery contains graphic images. Viewer discretion is advised.