Three hundred forty-three of its members were killed. Firefighters rushing in while everyone else was desperately trying to get out.
And no one city official showed his emotional devastation more publicly than fire commissioner Tom Von Essen, the man in charge.
The folks who were killed were his colleagues - and his best friends.
"I wasn't someone who came in from a federal agency to run the department," says Von Essen, who started as a firefighter. "I grew up in this organization, 30 years. To wind up as commissioner is the greatest job. And then to have the worst ending - it's the biggest thing to happen in my life."
If New York City's sorrow had a face after the terror attacks, it belonged to Tom Von Essen.
You could see it in his eyes at the mayor's news briefings and at the never-ending funerals - months of them.
You can still it in his eyes, masked by the passage of time. But just a little.
"I'll be driving and see someone who looks like that guy," says Von Essen, choking a bit. "I don't start crying anymore. I'm doing better with the crying."
But can he get over it? Has he gotten over it?
"No, no, no, I haven't - it's pretty obvious talking to you," he says, wiping his eyes. "You compartmentalize it like they tell you to do. And it helps you put it in one spot."
Von Essen mourned for his friends, of course, but he also mourned for his devastated department. His leadership team, who rose through the ranks with him, had been decimated.
And his department was criticized: If it had better communication equipment, many felt, firefighters would have known to get out - not keep going higher.
Von Essen doesn't buy that.
"A lot of the people didn't know what they were in for. But many people got 'Maydays' - they were told to get out. But they still went forward because people needed our help. They were aware of the danger - that makes the heroism more spectacular."
And perhaps most poignantly, Von Essen offers this: "They didn't know it was going to come down in 102 minutes. No engineer could have predicted that. If someone had said, 'Stop these firefighters 'cause these buildings will fall down,' we would have been known as the biggest cowards, not the greatest heroes."
Von Essen is not the public figure he once was. He's still involved with the FDNY through its non-profit foundation. And he'll be at the 10th anniversary ceremony, but as a guest, not an organizer.
"I'm not as involved with this as I would have liked to be," says Von Essen. "That, I can't do. That's a little too painful for me to deal with the politics and the families."
Standing at Ground Zero under renovation is also tough for Von Essen. But he says time will change how people view this site, once a field of death.
"This will be different," says Von Essen. "Wonderful. I guess 10 years from now, some may have forgotten about it. Maybe that's a good thing."
But will he have forgotten?
"Me? No, no," Von Essen says. "But I hope 10 years from now, I don't feel as whipped as I feel now. It's always there."