PHILADELPHIA (CBS) — There are new developments in the investigation of a fire and building collapse that left one Philadelphia firefighter dead and five others injured in Fairhill. The Bureau of Alcohol, Tobacco, Firearms and Explosives’ National Response Team has been deployed to Philadelphia to assist in the investigation.

“Anytime a firefighter loses his/her life in the line of duty, it is a tragic loss to the community,” Matthew Varisco, Special Agent in charge of ATF’s Philadelphia Field Division, said in a release. “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.”

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Philadelphia firefighters were called to a fire at a restaurant on the 300 block of West Indiana Avenue in North Philadelphia early Saturday morning. CBS3 has learned the upper floors were used as a residential space and the lower floor was occupied by a pizza parlor.

Fire officials were able to get the flames under control within 15 minutes and evacuate eight people living in the building.

Lt. Sean Williamson and five others went back inside the building to make sure there were no hidden fires and an hour later the building collapsed just before 3:30 a.m.

Sean Williamson, 27-year veteran of Philadelphia Fire Department

The collapse resulted in the death of Williamson, a 27-year veteran of the Philadelphia Fire Department. Williamson was most recently assigned to Ladder 18 in the Hunting Park section of the city.

Officials say it was a miracle that anyone was able to survive the building collapse.

Fire crews worked to rescue four other firefighters and an L&I employee. Firefighters Robert Brennan Jr., Dennis Daly, Lt. Sylvester Burton, Lt. Clarence Johnson, and Inspector Thomas Rybakowski were rescued from the rubble.

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ATF says the Philadelphia Fire Marshal’s Office requested assistance from the ATF’s National Response Team (NRT) due to the loss of life resulting from the building collapse. The NRT offers immediate and sustained nationwide response capability, typically deploying within 24 hours of being notified.

The team has access to state-of-the-art equipment and highly trained personnel who specialize in fire origin and cause determination. The NRT will be working closely with the Philadelphia Police Department and Philadelphia Fire Marshal’s Office to determine the origin and cause of the fire.

Philadelphia Fire Commissioner Adam Thiel was emotional during a press conference on Saturday.

“I’m almost speechless,” Thiel said. “I’m not done crying but I’ve cried once already.”

Thiel said Williamson was one of the most experienced lieutenants in the department. Lt. Williamson had trained countless numbers of new firefighters, and cadet firefighters and served in the U.S. Marine Corps.

“The worst thing for any firefighter or any medic, we all swear to save lives and protect property, and any time we can’t do that – whether it’s a resident of the city and certainly if it’s one of our own sisters and brothers, we feel it very deeply,” Thiel said. “This has been a tragic year for the Philadelphia Fire Department. We’re just absolutely devastated.”

He leaves behind his mother and son.

To honor his legacy, Mayor Jim Kenney says flags will be flown at half staff for 30 days in Philadelphia.

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