By Jack Fink

DALLAS, Texas (CBSDFW.COM) — On the 20th anniversary of the 9/11 terrorist attacks, former President George W. Bush paid tribute to the nearly 3,000 victims and the first responders and veterans who answered the urgent call of duty.

At the George W. Bush Presidential Center in Dallas, former President George W. Bush and First Lady Laura Bush hosted a screening of a new documentary by Apple and the BBC called 9/11: Inside the President’s War Room.

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“The lessons of 9/11 are as relevant today as they were 20 years ago,” the former president said.

He introduced three first responders in the audience, including retired New York City Firefighter Ron Parker. Days after the terrorist attack at Ground Zero, he was right behind President Bush when he spoke into the bullhorn.

Bush recounted the story that led up to the remarks that have been etched into the nation’s collective memory.

“I hear this voice behind me. He goes, ‘Louder.’ It was Parker. I turned to him and said, ‘This is as loud as it goes.’ Thankfully he started this movement, people started screaming where people said, ‘We can’t hear you,’ and I just blurted.”

That day, the president told those gathered around him amid all the destruction, “I can hear you. The rest of the world hears you, and the people who knocked down these buildings will hear all of us soon.”

In an interview before the event, Parker said it was what he and his fellow firefighters, police officers, members of the military and construction workers needed to hear.

“It was so reassuring. He showed up and he cared and let me tell you, the conditions were horrible,” he recalled.

Parker said he didn’t realize the full effect until watching a documentary 12 years later.

“I was like, ‘I interrupted the president of the United States and he answered me.’ I would think during one of his most famous speeches, it may have changed the course of his speech,” he said.

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Parker said he was looking forward to meeting the former president later in the evening.

He signed a copy of his memoir to give to Bush and included an apology.

Parker wrote, “It has been 20 years since I interrupted you at your greatest moment at Ground Zero. I now officially apologize to you for my bad behavior.”

In Shanksville, Pennsylvania, earlier Saturday, the former president honored the memories of the 33 passengers and seven crew on board United Airlines Flight 93, who fought back against the terrorists who hijacked their plane.

“In a sense, they stood for us all. The terrorists soon discovered a random group of Americans is an exceptional group of people,” he said.

The former Commander-in-Chief praised the heroic efforts of the first responders and millions of Americans who volunteered to serve in the armed forces.

“You have been the force of hope and mercy in dark places. You’ve been a force for good in the world. Nothing that has followed — nothing — can tarnish your honor.”

He remembered how a wounded nation found strength.

“The actions of an enemy revealed the spirit of a people. On America’s day of trial and grief, I saw millions of people instinctively grab for a neighbor’s hand and rally to the cause of one another. That is the America I know.”

But former President Bush portrayed a sharp contrast from what he saw 20 years ago and what he finds today.

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“Malign force seems at work in our common life,” he said. “That turns every disagreement into an argument and every argument into a clash of cultures. So much of our politics has become a naked appeal to anger, fear and resentment. That leaves us worried about our nation and our future together.”