'The Longest Shadow': 20 years later, 9/11 families seek justice -- and peace

Part 1 of ABC News' five-part series on the 20th anniversary of 9/11 looks at the ongoing questions of how the attacks could happen -- and who was responsible.

The sky was clear and blue. The gray towers stood, both guarding and welcoming, at the gateway to the nation. Out of nowhere came the impact, the blaze, the smoke -- and then the towers were gone. When the dust and flames finally cleared, a new world had emerged. The death and destruction defined that late summer day and remain seared in the minds of those who lived through Sept. 11, 2001. From the ashes and wreckage rose a new America: a society redefined by its scars and marked by a new wartime reality -- a shadow darkened even more in recent days by the resurgence of fundamentalist Islamist rule in the far-off land that hatched the attacks. Twenty years later -- with more than 70 million Americans born since the crucible of the attacks -- the legacy of 9/11 remains. From airport security to civilian policing to the most casual parts of daily life, it would be nearly impossible to identify something that remains untouched and unaffected by those terrifying hours in 2001. This week, ABC News revisits the 9/11 attacks and unwinds their aftermath, taking a deep look at the America born in the wake of destruction. "9/11 Twenty Years Later: The Longest Shadow" is a five-part documentary series narrated by George Stephanopoulos. Episodes will air on ABC News Live each night leading up to the 20th anniversary of the attacks, from Sept. 6-10. The series will be rebroadcast in full following the commemoration ceremonies on Saturday, Sept. 11.