PHILADELPHIA (WPVI) -- Federal and local investigators are probing a building collapse that killed one firefighter and injured several other people following a fire Saturday morning in Philadelphia's Fairhill neighborhood. The blaze broke out around 2 a. m. at a three-story restaurant and residential building on West Indiana Avenue. The fire had been put out, but then the building collapsed at 3:24 a. m. Lieutenant Sean Williamson, 51, was killed in the fire. 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. Officials say Lieutenant Sean Williamson, 51, was a 27-year veteran of the Philadelphia Fire Department. On Sunday, ATF National Response Team joined local authorities to begin a joint investigation into the fire. "Anytime a firefighter loses his/her life in the line of duty, it is a tragic loss to the community," said Matthew Varisco, Special Agent in charge of ATF's Philadelphia Field Division. "We are heartbroken for the families of these firemen and vow to work alongside our partners with the Philadelphia Fire Department and the Philadelphia Fire Marshal's Office to determine the origin and cause of this fire which resulted in the death of one firefighter and injuring of another. " Officials were back at the scene of 300 W Indiana Avenue in Fairhill today. ATF now joins Philadelphia on the investigation into the fire and building collapse that killed 27 year veteran, Lieutenant Sean Williamson. @6abc pic. twitter. com/NADMLzsMU3 — Bryanna Gallagher (@BGallagherTV) June 19, 2022 Williamson was a 27-year veteran of the Philadelphia Fire Department and was most recently assigned to Ladder 18 in the city's Hunting Park neighborhood. "It's going to be a rough few weeks coming up," 1st Deputy Fire Commissioner Craig Murphy said Saturday. EMBED > More News Videos One firefighter has died after being trapped in a building that caught fire in Philadelphia and then collapsed Saturday morning, fire officials said. Two firefighters and the L&I worker have been released from Temple University Hospital. Two firefighters remain hospitalized in critical but stable condition. The names of the injured members during the emergency response were: Lieutenant Sylvester Burton, Fire Marshal Lieutenant Clarence Johnson Firefighter Dennis Daly Firefighter Robert Brennan, Jr. Thomas Rybakowski, Supervisor, Emergency Services Unit, L&I EMBED > More News Videos Philadelphia Fire Officials give update on fire and building collapse in Fairhill "You can't predict this," Murphy told reporters at a news conference. "This was just a catastrophic accident that (has) really hurt our department. " Murphy said the building had been affected by the fire, but it was unclear what caused it to come down. Fire Commissioner Adam Thiel told reporters Saturday evening that rescuers "were able to communicate with" Williamson and another firefighter for most of the several hours they remained trapped, but because of the degree of the collapse and where Williamson was located within the structure "we were not able to save him. " The former Marine was "highly respected throughout our department" and had trained "countless" cadets, Thiel said. Williamson is to have a "full honors" fire department funeral "and given the outpouring of support that I've seen and we've seen as a department, you can expect this to be a pretty large event. " "We're absolutely grieving, we're mourning," Thiel said. "We have a lot more crying and a lot more processing to do this unfolds as we move forward with properly honoring Lt. Williamson," he said. Mayor Jim Kenney called it "a heartbreaking day for our city. " "For more than 27 years, he dedicated his life to serving and protecting the people of Philadelphia, and sacrificed his life protecting others," Kenney's statement said. "Early this morning, like every day, he exemplified heroism by doing what our first responders do every day: put on their uniform, leave their loved ones, and carry out their sworn duty to protect and serve the residents of this city. " The Associated Press contributed to this report. Report a correction or typo.