"They knew that this was going to be the toughest operation that they've ever been at and that they might not make it."
The next day, Cassano said it was right back to work. The process to rebuild the department continued with the ordering of equipment and trying to locate firefighters who were still missing.
"We were bloodied, we were battered, we were brought to our knees. But we never gave up hope," he said. "We knew we had hundreds of people missing and we knew we had to get to them as quickly as possible."
He said the department's members were working 80- to 100-hour weeks in many cases.
"Either you were working at the World Trade Center site or when you worked your tour of the firehouse you came right back to the site," he said. "The department was always close. It's like a second family . . . I think that helped us tremendously."
In the years following the 9/11 attacks, much has changed for the FDNY; from turnover in personnel to enhanced training to upgrades in technology. While it is still a department in mourning, it is also a department that is moving on.
Five years ago, a state-of-the-art operations center was unveiled that allowed capabilities Cassano didn't have at his disposal on that tragic day.
"Sept. 11 made us look at how we operate and rethink some things," he said.
The remaining threat of future attacks was still very present while the department worked to rebuild and still is to this day. On May 1, 2010, firefighters responded to what appeared to be a run-of-the-mill car fire that turned out to be an attempt to detonate a bomb rigged to the vehicle.