ARCHIVAL (9-11-01):ANCHOR: This just in, you are looking at obviously a very disturbing live shot there.

ARCHIVAL (NBC NEWS, THE TODAY SHOW, 9-11-01):KATIE COURIC: A plane has just crashed into the World Trade Center here in New York City.

ARCHIVAL (CBS NEWS, 9-11-01):BRYANT GUMBEL: We dont know anything more than that, we dont know if it was a commercial aircraft.

ARCHIVAL (ABC NEWS, 9-11-01):LIVE COVERAGE: Oh my God! Another plane has just hit right it hit another building. Flew right into the middle of it.

ARCHIVAL (CNN, 9-11-01):LIVE COVERAGE: The building is exploding now. You got people running up the street.

ARCHIVAL (9-11-01):AUDIO OF A VOICEMAIL FROM A WOMAN INSIDE THE WORLD TRADE CENTER: Sean, its me and I just wanted to let you know I love you and Im stuck in this building in New York. I just wanted you to know that I love you always.

ARCHIVAL (FOX NEWS, 9-11-01):ANCHOR: I think we have a terrorist act of proportions that we cannot begin to imagine at this juncture.

MARK DEMARCO: If a popular novelist sat down and wrote a novel, people wouldnt buy this book because it would be so outrageous. This could never happen.

DAN MCNALLY: I dont know what to tell young people about that day. I know when I die, the first thing I want to ask St. Peter is like, hey, what happened? How did this happen? Cause I still have questions.

TEXT ON SCREEN: SEPTEMBER 11, 2001

DETECTIVE DAN MCNALLY, RETIRED (NYPD BOMB SQUAD): It was my day off. And it was a beautiful day, you know, really beautiful blue skies.

OFFICER MARK DEMARCO, RETIRED (NYPD EMERGENCY SERVICE UNIT TRUCK 1): It was going to be the morning of the Mayoral Primaries. They switched me to do election duty that day.

OFFICER BILL BEAURY, RETIRED (NYPD EMERGENCY SERVICE UNIT TRUCK 7): Like every other morning, we were sitting around the table just going over what was going on with the kids, with the family.

DAN MCNALLY: I was in my bathroom taking a shower and I had a window that faced south and I heard what I thought was an explosion.

NARRATION: At 8:46am, American Airlines flight 11 crashed into the World Trade Center North Tower, building one. It was one of four planes hijacked by 19 Al Qaeda terrorists that day. At first, it seemed a tragic accident an airplane off course.

DAN MCNALLY: As Im watching this, trying to figure out do I have to go to work? The second plane hit and I saw it on TV. And that was it.

MARK DEMARCO: And all I seen was this huge fireball coming out of tower two, out of the South Tower. And for a second I, I didnt know what it was. I thought maybe just the building blew up.

DAN MCNALLY: I ran downstairs and Im looking south and both towers are on fire. And it was stunning.

NARRATION: With the second catastrophic strike, it became clear that the country was under attack. New York City mounted its largest rescue effort in history, dispatching more than two thousand firefighters, medical workers and police officers to lower Manhattan.

MARK DEMARCO: It was a little hard getting through the streets because there was debris all over. Stuff that had come out of the building from the plane hitting it.

DAN MCNALLY: You gotta remember the north tower is fully engulfed. Itspeople are jumping out of the windows. And you can hear em hittin the deck.

MARK DEMARCO: There was this these popping sounds going off. We realized it was people that were either jumping off the building or were falling out of the building.

DAN MCNALLY: I did not look. I was not gonna look. I am very protective of my psyche. I only need to see what I need to see in situations like this. But you couldnt close your ears.

NARRATION: Rescue workers rushed in to help the more than 16,000 people still inside, putting their own lives on the line. Officers Bill Beaury and Mark DeMarco and their team entered the North Tower, tower one.

BILL BEAURY: We ended up at stairwell C, and we started our climb up.

MARK DEMARCO: Were running into other people. I said, Follow just follow the wall down. When you get outside, try not to panic, I said, But run. I said, Get away from this site as fast as you can. All of a sudden the lights went out, the building went dark, ceiling panels were falling down.

BILL BEAURY: It sounded like a freight train going by.

MARK DEMARCO: And, and the whole place was shaking like an earthquake. With this our radio started getting very active.

ARCHIVAL (9-11-01):RADIO CALL: About 15 floors down from the top. It is totally glowing red on the inside. It is inevitable. I dont think this has much longer to go.

NARRATION: At 9:59am the 110 story South Tower, tower two, crashed down in just 10 seconds.

MARK DEMARCO: And thats when a little bit of shock started setting in. Because we knew what these buildings looked like. We knew how, how long it took them to put them up. And for something like that to come down in a matter of seconds, it was just mind boggling. You, you couldnt wrap your head around it. One of the officers, who was outside, he put over the radio he said, Everybody who is in Tower One, evacuate it immediately. Get, get the hell out of there.

NARRATION: DeMarco and Beaury, along with Sgt. Michael Curtin and Officer John DAllara saved civilians from certain death, ushering them from the stairway and lobby. The men then made their way from the North Tower to the adjacent building number six. There they met McNally and his partner, Claude Danny Richards. Then only moments later

ARCHIVAL (9-11-01):NEWS REPORT: One plane colliding with one Twin Tower. Oh my God.

ARCHIVAL (9-11-01):RADIO CALL: Its collapsing. Its collapsing. All units watch out it is collapsing. All units the North Tower is coming down.

ARCHIVAL (9-11-01):NEWS REPORT: Oh my God.

DAN MCNALLY: And we put our heads down and we held onto each other and we tried to become as small as we possibly could.

MARK DEMARCO: And I just I just laid there. And I remember saying goodbye to my wife and kids.

BILL BEAURY: And I remember seeing all four of my kids, one after another.

DAN MCNALLY: I had time to get out one prayer. And it was, Dear God, if youre gonna take me please make it fast. You know? Thats how scared I was. I was terrified.

MARK DEMARCO: I remember saying to myself I said, Is this death? Is this what death is?

DAN MCNALLY: And I was waiting for like a piece of steel to come through my back and out my chest or something.

NARRATION: The 110 story North Tower landed on building six blasting a crater through it, only a few feet from where McNally, Beaury, and DeMarco were huddled.

DAN MCNALLY: Youve probably seen the large cloud dust. Well, we were in it. I had a respirator. And that became very clogged very quickly. And I couldnt breathe. And you start really panicking when you cant breathe. All of a sudden it was silent. And youre like, wow. I survived.

MARK DEMARCO: I started moving my fingers and my toes. And then I said, Oh, is it I guess thats a good sign. I figured I got my limbs and I figured I was alive.

BILL BEAURY: I dont know how long we were there or how long we were in that spot, but we ended up getting up, and we all met up.

MARK DEMARCO: Visibility was still horrible. Little, little patches of fire were breaking out. And I said to myself, I said, The building collapse didnt kill me. I said, Now, now Im going to burn to death. Bill Beaury yells out. He goes, I think I found a way out. I think theres a hole over here. A hole in the building.

DAN MCNALLY: And we got this conga line and were gonna try to get out. It was Mark DeMarco, me and Billy behind me. You could still see our handprints on the wall as we were guiding ourselves blindly.

MARK DEMARCO: We were able to climb out the windows out to the outside mezzanine level, which was facing West Street. The bridge was all collapsed. Everything was dead silent. No cars. No birds. Nothing.

NARRATION: McNally, Beaury, and DeMarco searched in vain for three NYPD officers who moments before had been right there with them, a search that would continue for months on end.

DAN MCNALLY: They couldnt find Mike Curtin, John DAllara or Danny Richards. And we didnt find em until the following spring. They were buried under the sheer weight of the World Trade Center. And they couldnt have been more than 20 feet behind us when the tower came down. And that was the focus of the rest of my days there was making sure that people knew exactly where I thought these guys were, you know? So that we could recover their bodies.

MARK DEMARCO: And to this day, I still, I, I dont understand how, why Im still here. Theres a guiltiness about it because theres a, theres a lot of them that were more talented. Theyre just better people than me. And they didnt make it. I was able to come home. I was able to see my family. I was able toI was able to see my kids grow up.

DAN MCNALLY: Im fortunate. Im lucky. You know, I thank God that Im still breathing, but I inhaled a lot of stuff and, you know, sometimes thatI dont think about it, but I know there might be a ticking time bomb in my chest, you know?

NARRATION: To this day only 60 percent of the victims have been identified most only through traces of DNA leaving more than 1,000 families without any remains of their loved ones to bury and mourn.

DAN MCNALLY: I miss my friends. Even the ones who survived. Theyre not the same. Theres a lot of guys who are comin down sick with cancers that are horrific.

MARK DEMARCO: I think about 9-11 every day. I mean, its, its not, it doesnt consume me where I, I feel I need to go see a shrink or, or seek help somewhere. I just dont ever want to forget it. I dont forget it. I wear a memorial bracelet with the, the guys who perished that day from my unit.

MARK DEMARCO (READING FROM HIS BRACELET): Sergeant John Couglin, Sergeant Michael Curtin, Sergeant Rodney Gillis, Detective Joseph Vigiano, Police Officer John DAllara, Police Officer Vincent Danz..

BILL BEAURY (READING FROM HIS BRACELET): NYPD, ESU, Truck 7, 9-11-01.

BILL BEAURY: And theres only been one time that this hasnt been on my arm. In 2013, I had open-heart surgery.

DAN MCNALLY: What I always try to tell people about that day is that there were so many heroes that day. So many heroes. Helping each other down the stairs and out into the lobbies. It was a day when we needed heroes and many of em showed up. Did we get everything right? Probably not. Did we do the best we could? Yeah.

TEXT ON SCREEN:

2,753 people perished in the attack on the World Trade Center.

Among the dead were:

23 officers of the New York City Police Department

37 officers of the Port Authority of New York and New Jersey

343 members of the New York City Fire Department

The attacks on September 11, 2001 were the deadliest ever on U.S. soil.