20 haunting photos from the September 11 attacks that Americans will never forget

It's been 15 years since the attacks of September 11, 2001. But for millions of Americans, haunting memories of that day are still fresh, and many lives were changed forever.

On 9/11, terrorists hijacked four planes and were able to crash two of them into the World Trade Center's twin towers in New York and one into the Pentagon.

The remaining jet crashed in Pennsylvania when passengers overpowered the hijackers.

A day that started out with clear blue skies ended with a mass of twisted, smoldering metal where the Twin Towers once stood, leaving 2,977 people dead in New York, Virginia, and Pennsylvania, along with the 19 hijackers.

September 11 happened over a decade ago. The Washington Post's Aaron Blake tweeted that a quarter of Americans are too young to remember it. I have three kids who have no memory of it at all - they weren't born yet.

But we certainly don't want to forget, even as we get on with our lives. So in memory of that day, here are 20 images that capture what no American should forget. And as someone living in New York City at the time, here's what it was like to witness the tragedy as it unfolded.