The city's fire department suffered crushing losses on 9/11 and in the 15 years since, the agency has built itself back up, but it hasn't been easy, and it hasn't been without looking back. NY1's Amanda Farinacci filed the following report.

For FDNY Commissioner Daniel Nigro September doesn't just mean the end of summer. It begins a month of grieving and reflection, for all that was lost on 9/11.

"It'll never be part of our past. It'll always be part of the department. So it's a tough week for us and particularly September 11th is the toughest day of the year," says Nigro.

A total of 343 firefighters were killed on 9/11 - many of them Nigro's friends and co-workers.

At the time, he was the department's second highest ranking officer, the Chief of Operations. He responded to the World Trade Center with his friend, Chief of Department Pete Ganci.

Ganci died, and six days later, Nigro was tapped to fill his job.

After retiring in 2002, Nigro came back to the department as its commissioner in 2014.

"This department is a proud department, a well trained department, and we're ready for any and all eventuality," he says.

Forty-four hundred years of experience was lost on 9/11, but the FDNY has taken great strides to fill its ranks since.

More than 7,300 firefighters have been hired. The rest of the department's 11,000 members were working on 9/11 and remain active today.

Firefighters now receive four times the training they did before the attacks, on things like active shootings and natural disasters.

Technology has greatly improved like inside its command center and in the field. Radios now allow communication between police officers and firefighters.

"The radios continue to advance. The problems that we had with the radios 15 years ago have been solved," says Nigro.

The FDNY shares information with other city agencies, using NYPD and DOT cameras to watch incidents as they unfold in real time.

On the wall in the lobby of FDNY headquarters is a memorial for the firefighters who have died of 9/11 related illnesses.

More than 120 names adorn the display - losses Nigro sees as a second wave of sorrow for his department.

And he, too, suffers from diminished lung capacity from his work at the site.