Heaps of bricks, twisted metal, and splintered wood spilled across Third Street and Indiana Avenue in Philadelphia’s Fairhill neighborhood Sunday morning, the battered sign for Star Pizza & Seafood peeking from the rubble.

What caused the fire in the mixed-use building and what ultimately led to its collapse early Saturday — killing Lt. Sean Williamson, injuring four other firefighters and a city building inspector, and displacing 11 residents — could take a week or more to uncover, fire marshals said.

“This will be painfully slow: a systematic effort, piece by piece,” Fire Marshal Battalion Chief Christopher Beale said at the scene Sunday morning. Officials will examine all possible ignition sources and rule out possible causes, one by one. For now, “they’re all on the table.”

Questions also remain over how a pizza shop and apartments were developed at the site without any city permits or licenses being issued.

Fire officials will also work with the city’s Department of Licenses and Inspections to assess what contributed to the collapse of a three-story building that officials said was the site of unpermitted construction in recent years.

“If the structure is weak to begin with, and then the fire attacks it, it weakens it. The fire might have just been the catalyst,” Beale said.

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City officials and neighbors alike described the blaze as a minor one that had already been declared under control when the building collapsed shortly before 3:30 a.m. Saturday. Three firefighters and one L&I inspector were initially trapped but quickly freed. It took nearly four hours for rescue workers to reach Williamson and another firefighter, Robert Brennan Jr., who was taken to Temple University Hospital. Brennan and firefighter Dennis Daly remained hospitalized in serious condition as of Sunday afternoon, a city spokesperson said.

According to Beale, Williamson and others were in the process of what’s known as “overhauling.”

That’s the necessary step of checking a scene for any lingering fires that may be smoldering above ceilings or behind walls. “You don’t want to come back twice,” he said, noting that a return trip would likely mean even more serious damage. At the same time, he said, investigators had already begun working to identify the cause of the fire on Saturday when the building came down.

Neighbors said that work at the building had included moving an exterior staircase and, apparently, construction on the upstairs apartments. Though three families were living in the building, there were no active rental permits, city records show. Those records also show building code violations at the site, many of them related to fire safety issues, dating to 2019, when the business was under different ownership. An inspector who returned in June 2021 found the restaurant closed and the space unoccupied, a city spokesperson said, “so no further action was warranted.” City records show no current business licenses for a restaurant at that location.

Khalil Al-Ashraf, the owner of Star Pizza, hung up the phone when contacted by a reporter on Sunday morning.

Bernard Torres, who lives across the street, said he was awakened by his brother warning of the fire around 2:30 a.m. “It didn’t look that serious. They stopped the fire in, let’s say, 15 minutes. We went back to sleep, and then we heard a noise. Boom!”

Smoking a cigarette on his porch, feet away from the pile of debris, he summed up the scene in a word: “Devastating.”

A perimeter of caution tape and rows of red Fire Department vehicles ringed the scene Sunday morning, where no active work had yet begun.

“I’m sorry for you guys’ loss,” a driver called to department staffers as he was diverted down American Street.

Williamson, 51, who was assigned to Ladder 18 in Hunting Park, is survived by his mother and a son. His mother, Barbara Nerch, referred questions to the Fire Department. A spokesperson said funeral plans had not been finalized.

“Sean had a big family, a loving family. That’s all I can say. He’s going to be very missed,” Nerch said.

Williamson’s public service elicited words of praise and gratitude from Mayor Jim Kenney.

“For more than 27 years, he dedicated his life to serving and protecting the people of Philadelphia, and sacrificed his life protecting others,” Kenney said in a statement.

Police Commissioner Danielle Outlaw tweeted that the Police Department “is heartbroken as we mourn the loss of our fallen @PhillyFireDept brother” and thanked the Fire Department “for everything they do to keep our city safe.”

Dave Skutnik, regional communications director for the Red Cross of Southeastern Pennsylvania, said the organization was providing emergency assistance for three families, totaling 11 people, who were living in apartments above the pizza shop.

”We provide them with financial support for temporary lodging, food, and clothing and they will work with our caseworkers on an individual basis to find new places to live,” he said.