The flames broke out just after 5:30 a.m. on the 2000 block of S. 4th Street, near McKean Street.
Heavy smoke was seen when firefighters arrived. However, the smoke quickly escalated into uncontrollable flames.
Crews rescued some occupants on an upper floor. A woman was taken to the hospital with smoke inhalation and was in stable condition.
There is no word on the severity of their injuries.
The building contained 11 units. Residents claim they heard no fire alarms, but were awoken by police and firefighters banging on their doors to evacuate.
The fire was placed under control at approximately 7:17 a.m.
Residents say they didn't hear any fire alarms, and were only roused from their sleep by police and firefighters banging on their doors.
One woman, Nicole Kolbach, initially believed she lost her cat in the fire. The pet was later found in the basement alive. She said it was impossible to see as the fire raged.
"When you opened your eyes your eyes were burning because the whole building was filled with smoke already. There was no flames, there was just smoke," Kolbach said. "The next thing you know we had a big explosion."
That explosion was from a PECO transformer that blew up during the fire.
The Red Cross temporary shelter at the Horace Furness High School had to be evacuated due to excessive smoke. It was relocated to Sharswood Elementary School, located at 2nd and Wolf Streets.
By Thursday afternoon 17 people, including five children remained at the shelter.
Classes at Furness H.S. are cancelled today due to smoke.
There's no word on a cause of the fire at this time. However, investigators say the do not think it is suspicious.
The building will have to be torn down.