Lester Holt reports on how the investigation into the murder of four-year-old Barbara Jean Horn revealed decades of misconduct across Philadelphia's criminal justice system.

On the morning of September 11th, 2001, 40 men and women boarded a flight at Newark Airport bound for San Francisco. They were businessmen and women, retirees going on vacation, students headed home to school. Ordinary people on what seemed like an everyday flight. But the passengers and crew of United 93 would soon become part of the worst terrorist attack in American history. And their actions that day, banding together to fight the terrorists, would inspire the nation.

Our first full report on this incredible story aired in early October 2001. With the country still reeling from the 9/11 attacks, family members introduced us to the heroes of Flight 93. They recounted heartbreaking final phone calls home and the discovery of goodbye messages on answering machines. And they told a story of impossible bravery: how the passengers and crew took a vote and tried to take back the cockpit from the four hijackers in the face of almost certain death.

It's been 20 years since United 93 went down in a Pennsylvania field. This past summer we met up with some of those families again. They told us how they remember that awful day and how they've coped with their loss. They also brought us up to date on their lives today, 20 years later. It's a story of remarkable resilience.