A "Waves of Flags" display at Pepperdine University in Malibu, California, on Monday. The annual display of 2,997 flags commemorates each of the victims of the Sept 11, 2001, terrorist attacks on American soil.

On this day 17 years ago, 19 Al-Qaeda militants hijacked four planes, flying two into the twin towers of the World Trade Center complex in New York and crashing a third into the Pentagon in Virginia. A fourth plane smashed into a field in Pennsylvania.

The shocking attacks led to the launch of deadly, long-running wars in Iraq and Afghanistan - countries that remain plagued by violence and instability to this day.