"What started as a normal, beautiful, blue sky kind of a day spiraled into a day of death and chaos that would almost feel like it would never end."

filmmaker Gédéon Naudet

On September 11, 2001, two filmmakers making a documentary about a New York firefighter found themselves instead documenting the attacks in Lower Manhattan. Right after the first plane crashed, Jules Naudet followed FDNY rescue crews into the burning North Tower of the World Trade Center. His brother Gédéon Naudet filmed back at the firehouse before making his way to Ground Zero. The video they captured would later become a documentary: "9/11." The brothers, along with retired firefighter and filmmaker James Hanlon, spoke to CNN about their experiences.

Their footage -- raw, chaotic and nearly inconceivable -- offers a rare look into a day that continues to haunt the nation 15 years later.

8:46 a.m.

First plane hits the North Tower

I know I need to keep filming, something is happening. But what ...
Jules Naudet

Fire crews are on a call to a gas leak near the World Trade Center when the first plane hits. Jules Naudet and Chief Joseph Pfeiffer speed to the scene, as they had every day that summer, unaware of how deadly this day would be.

Everything goes in slow motion. Sound, images, suddenly time stops.
Jules Naudet

In the North Tower lobby, fire chiefs set up the command post – while bodies and debris hit the ground outside.

We have no idea what happened ... You can be 10,000 miles away in front of a television and you knew more of what was happening than we did.
Jules Naudet

9:03 a.m.

The second plane hits the South Tower

If there's a death at a fire, if you couldn't save someone ... that's always really tough to take. And in 9/11, that was times a thousand, times a million because everyone wanted to help, everyone wanted to do something, but the loss of life had already taken place.
James Hanlon

9:59 a.m.

The South Tower collapses

The sound, the rumble, gets louder and louder and I know it's death coming for me. I know it. There's no point in running.
Jules Naudet

Battalion 1 and Jules Naudet experience the South Tower collapse, from inside its twin.

Pfeiffer and Naudet emerge from the North Tower to an apocalyptic scene. First responders had 29 minutes to make a choice: help those trapped in the North Tower or retreat to safety before it came down as well.

To see braveness, to see courage right in front of you – for me has more of an imprint than the fear experienced on that day.
Gédéon Naudet

10:28 a.m.

North Tower collapses

A wall of smoke and debris overtakes anyone running on the streets below.

Then the noise stopped, and I'm still alive. And I think it took a few seconds to realize that I'm going to continue living. I was so sure I was going to die.
Gédéon Naudet

The dust eventually settled, but the city would breathe it in for months.

People were afraid to ask: ‘Have you seen so and so? Have you seen Mike? Have you seen Steve? Have you seen Mac?' Because they were afraid somebody might say no.
James Hanlon

Everyone had a story to tell on 9/11 -- where they were, who they lost. Lives forever changed, with that day echoing into the weeks, months and years that followed.

It's an extraordinary tale … of resilience, of survival, of courage, of love. For me, this is the legacy of 9/11.
Gédéon Naudet