The fallen firefighter was not immediately identified, but Deputy Fire Commissioner Craig Murphy said he was a 27-year department veteran. "It's going to be a rough few weeks coming up," Murphy said at a media briefing around 8 a.m.
The building had caught fire just before 2 a.m. Saturday, Murphy said. The fire had been put out, but then the building collapsed at 3:24 a.m.
Four other firefighters and an inspector with the city's Department of Licenses and Inspections were also trapped at the time of the collapse. One firefighter jumped from the second story of the rubble, Murphy said.
The others were pulled out at various times. They were taken to the hospital and said to be in stable condition. The inspector has since been released. The firefighter who died was pronounced dead at the scene.
Numerous firefighters were standing nearby as the rescue effort unfolded, and some were seen hugging or wiping tears from their eyes, multiple news outlets reported.
"You can't predict this," Murphy told reporters at a news conference. "This was just a catastrophic accident that (has) really hurt our department."
Investigators were looking into what caused the collapse. Murphy said the building had been affected by the fire, but it was unclear what caused it to come down.
Mayor Jim Kenney said in a Twitter post that he was "grieving with the members of the Philadelphia fire department and all Philadelphia who lost one of our own in the line of duty today."
Patricia Sermarini told The Philadelphia Inquirer that she rushed to the scene when she saw the alert about the collapse because her son-in-law, a firefighter, was on the morning shift. She said he had been one of the firefighters on scene but had made it out of the building just before it collapsed.
But moments later, Sermarini said, she saw firefighters pull a body out from the rubble.
"It's so terrible," she said. "This is so hard for them. They just want to get home to their families."