Nine firefighters were killed in Charleston, South Carolina, after the roof of a burning furniture store collapsed, city officials confirmed Tuesday, according to dpa. The fire alarm went out at 7 pm Monday, and the firefight were were on the scene almost immediately from a nearby fire station, Mayor Joe Riley said in broadcast remarks. The rescue workers were trying to rescue a trapped employee. The deaths were the largest tragedy for US fire and rescue workers since the 2001 terrorist attacks, when 343 firefighters died in the collapsing World Trade Center towers. "The firefighters are a family unit and they are grieving," Riley told CNN. "We are going to be working close with ... the families." He said the incident was "being investigated" and it was important "not to make assumptions" until the facts were known. Arson was not suspected, he said, according to Bloomberg financial news service. "Nine brave, heroic, courageous firefighters of the city of Charleston have perished fighting fire in a most courageous and fearless manner," Riley said. The one-story building had no sprinklers, fire and rescue spokesman Lieutenant Chad Watson said in a broadcast interview. Television footage showed weeping fellow firefighters, standing at silent attention, their hands on their foreheads in a salute, as the bodies of their comrades were carried by. The southern Atlantic coastal city employs 237 firefighters.