In a powerful tribute, thousands of drones illuminated lower Manhattan to form the images of the Twin Towers -- one light for each person lost on the day of the September 11th attacks. David Muir shares how the breathtaking drone show came together in collaboration with members of the 9/11 community.

ABC News Chief Investigative Correspondent Aaron Katersky reports on the ongoing health challenges faced by Sept. 11 first responders and details the security measures in place at Ground Zero.

A quarter of a century after the horrific terror attack on Sept. 11, 2001, Americans' grief remains, but hope endures – and so does the promise: never forget. David Muir reports on the day that forever changed America and the world.