September 11, 2001 remains one of the most shocking events in world history, changing the world forever. In this first of a two-part documentary marking the 25th anniversary of the tragedy, rescue workers, survivors and witnesses relive the events of 9/11, revealing how a regular morning turned into an unimaginable nightmare. Some tell their stories for the first time, describing how chaos and fear grew minute by minute.

Within less than an hour, three hijacked aircraft had struck New York’s iconic Twin Towers and the Pentagon. But the nightmare was far from over. As a global audience watched in horror, the Twin Towers collapsed, the Pentagon continued to burn and, in western Pennsylvania, a fourth hijacked airliner, United Flight 93, crashed into a field. Amid the devastation and fear, extraordinary acts of heroism emerged. Rescue workers, survivors and witnesses remember the tragedy and the moments that changed the world forever.