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Watch the Dateline episode "Heroes: The Story of Flight 93" now

Lester Holt reports.

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.

We also gathered together some of the children of Flight 93 for the first time. Many of them had never met before, but it was clear they have a special bond and a commitment to sharing their parents' heroism with future generations.

There were so many Americans who had to make brave decisions on 9/11. F-16 fighter pilot Heather Penney accepted what she thought was a suicide mission that morning. She didn't know about Flight 93, specifically; still, with reports of hijacked planes headed for Washington, D.C., she took to the skies to protect the nation's capital. But there was one big hitch: She had no weapons on her jet, which meant the only way to take out a hijacked plane would have been to ram into it.

At 10:03 a.m. that morning, the hijackers drove United 93 into the ground at the site of a deserted coal mine near Shanksville, Pennsylvania. Today, the site has been turned into a memorial to the flight. There is a museum and a marble wall honoring each of the 40 heroes. Many of the families will be on site to commemorate the 20th Anniversary.

President George W. Bush called what the passengers and crew of Flight 93 did that day one of the most courageous acts in American history. They were 40 ordinary people who did the extraordinary when it counted most. They sacrificed themselves so that others might live.

You can watch our full episode, "Heroes: The Story of Flight 93" now.