Officials say 40-year-old Mike McGuire of Engine 27 walked up 11 flights of stairs at Norman Blumberg Apartments to help trapped residents.
However, during the rescue attempt, McGuire was using a fire department air pack and somehow ran out of air. Normally, there is a warning alarm to alert users of a low supply. It is not known if the pack malfunctioned.
McGuire, the father of one, was found unconscious and had to be rescued himself.
McGuire remains at the Hospital of the University of Pennsylvania and is being treated in a hyperbaric chamber to reduce carbon monoxide levels in his blood.
Mayor Michael Nutter, who visited the injured firefighter, says McGuire has undergone several cycles of oxygen therapy inside the hospitals hyperbaric chamber.
He was transported in critical condition, but was upgraded to serious late Wednesday night.
"I pray to God that he's all right, because they were on the job in here. They were on the job," said resident Vetima Ruffin.
McGuire was one of 10 firefighters injured.
Eight of his fellow firefighters were treated and released from area hospitals, however, along with McGuire, a battalion chief remains under observation.
In addition to the 10 firefighters, a woman and two children, a boy and girl, were also injured in the blaze. The three residents were said to have suffered non-life-threatening injuries.
Investigation led officials to determine a faulty extension cord in apartment 805 caused the fire, Philadelphia Fire Commissioner Lloyd Ayers told the press during an afternoon news briefing. Authorities believe the extension cord was possibly connected to a phone charger, a laptop computer, and something else that they would not identify.
About 105 people were inside the building at the time the fire started. Some people were trapped on the floors above the fire and rescuers had to evacuate them through the fire towers.
"We couldn't get out, we ran to the 13th floor with a wet towel but the smoke pushed us back up," said resident Janice Brown. "So we ran back upstairs and we got on the balcony and started swinging our towels, telling people we're stuck up here."
Residents who were rescued told Action News that the firefighters kept them calm as they helped them down the dark, smoke-filled hallways and stairwells. Some residents found they couldn't get out so they went to their balconies and waved towels hoping they would be saved.
Soon, firefighters arrived and helped them down. Commissioner Ayers said, "The firefighters were in one mode - that's rescue mode." Residents called the firefighters who saved them "heroes."
Some children were trapped in the apartment adjacent to where the fire started. Firefighters intentionally kept them in the apartment during the battle against the blaze because they didn't have breathing apparatus for the kids. Once the smoke cleared enough they were allowed to leave. The children were terrified, but otherwise fine.
One of those children was Diamond Morris, who said "There was smoke coming under our door and I started crying. I couldn't breathe." Troy Sweet, who lives in 802, the same floor where the fire started, said he went to the store for a few minutes on Wednesday morning and, when he got back, he saw his six kids screaming for help.
"My kids were screaming off the balcony screaming 'Dad, dad, dad, dad.' I go to run in and try to get them," Sweet said. "My family is in there, I'm going to go in there and die with them."
The Red Cross provided flood, blankets, and mental health counseling for those affected at a shelter at John Reynold Junior High School across the street from the building.
Flames were first reported inside the 18-story Blumberg Apartments at 24th and Jefferson streets at about 5:50 a.m.
Flames were bursting out of the windows of one of the apartments on the 8th floor of the building as firefighters arrived on the scene.
The Philadelphia Fire Department struck a second alarm for the blaze just after 6:20 a.m.
By 6:45 a.m, large puffs of white smoke poured from the windows where the fire was burning, showing that firefighters were able to get water on the flames.
Wind worked against firefighters the entire time. Strong winds blew the smoke back toward the building during the entire fire fight. Commissioner Ayers said, "The wind was out of the west and pushing the smoke back on the firefighters the entire time."
The fire was placed under control around 7:06 a.m.
As of Wednesday night, most of the residents were allowed to move back to their apartments. Only 11 units remain unlivable.
The building is run by the Philadelphia Housing Authority.