Williamson, was found in the collapse at 6:45 a.m, authorities said.
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.”
“I am deeply saddened to learn of the tragic death of Philadelphia Firefighter Lt. Sean Williamson. This is a heartbreaking day for our city … 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. I share my deepest condolences with everyone who knew and loved Sean. I ask all Philadelphians to keep his family, and all members of the Philadelphia Fire Department, in your prayers at this difficult time,” said Kenney.