Twenty years have passed since the terror attacks of Sept. 11. It’s a milestone to be sure. But no amount of time can fade my memories of that day.

What I remember most about 9/11 is stepping off the helicopter at Shanksville, Penn. – and being met by the brutal sound of silence. Emergency personnel searched the fields. Ambulances were at the ready. Rescue workers wanted someone to save.

But the passengers and crew of Flight 93 – 40 heroes strong – were the first, first responders on the scene.

They had already run toward the danger. They had already taken up the battle. And they were already in the arms of God.

A wonderful group called Friends of Flight 93 National Memorial has created a Heroes Award in their honor so their story is never forgotten. Could anything be more appropriate?

Above a Pennsylvania field, and the Pentagon’s stone and the once-towering World Trade Center, we lost nearly 3,000 souls from more than 80 nations. We lost them too terribly and too soon.

And yet, despite the weight of pain and anguish on our shoulders, we pulled together.

Do you remember? Some of you brought foil-covered plates of food to firefighters. Others held candlelight vigils in cities, large and small. Stores ran out of flags. Schools and communities raised money for grieving families. On the steps of the Capitol, members of Congress sang “God Bless America.”

At our country’s worst moment, we survived on a steady diet of kindness, generosity and compassion. You may not find those words in any national security plan. But I can assure you – those concepts are just as critical to our national resilience as any component of national defense.

News can be overwhelming

I know the country seems fractured at the moment. And that the daily news headlines seem too much to bear. Some of you have told me you’re feeling overwhelmed by the challenges we face and uncertain about our ability to meet them.

But I would ask you to remember: Our shared values, our shared responsibility to one another and the country we all cherish – that’s been the hallmark of the American story for the past 20 years, for the past 245 years.

Even in these past 20 months, doctors, nurses, teachers, grocery clerks, truck drivers, people everywhere, have pulled together to keep our economy moving, our students learning and all of us healthy and safe.

We are a nation of more than 333 million people – of many colors and cultures, of many religions and political beliefs. But do you remember? We’re also a nation of Rosie the Riveters. Of Live Aid concerts and charity telethons, community bake sales and clothing drives. We’re the hearts and billfolds that open daily for the vulnerable among us – our elderly neighbors, the hungry, the homeless, victims of hurricanes and earthquakes.

That’s who we are. That’s in America’s DNA.

Common humanity unites us

We know that our humanity toward one another is our saving grace. We know this – not because we were always good to each other in the past, or because we’ve always been a truly United States.

We know it because, at times, we’ve strayed from that humanity, that empathy and that unity. We’ve learned from the consequences of our mistakes that America is not perfect – so we try harder and strive to be a more perfect union.

The late Sen. John McCain was a dear friend of mine for decades. “Do not despair of our present difficulties,” he said in his farewell message, “but believe always in the promise and greatness of America, because nothing is inevitable.”

John took nothing for granted. He fought every day of his life for the greater good. For the country he loved. For the cause he believed in most – service above self.

I’m profoundly grateful for the opportunities I was given to serve my country. From soldier to secretary, I’ve seen America on its worst days and its best. I’ve seen people give all they had to give. And I’ll never forget it.

I’ll never forget the silence on a Pennsylvania field one September morning.

Or the sacrifice of 40 heroes strong.

I’ll never forget all of those we lost 20 years ago. Too terribly and too soon.

May we keep their memories close – as well as each other.