Philadelphia mayor said the rowhouse was owned by the Philadelphia Housing Authority.
Thirteen people, including seven children, died when a fire tore through a converted three-story house in the eastern US city of Philadelphia on Wednesday, officials said.
Philadelphia Fire Department deputy commissioner Craig Murphy said the death number was "dynamic because there is still an ongoing recovery effort inside."
Two additional people were rushed to hospital, he added.
The fire officer said that it was the worst fire he had seen in 35 years on the job.
Murphy said it was too early to say what caused the blaze but that his department was conducting an investigation.
"It's not necessarily considered suspicious but we have all hands on deck because of the magnitude of this fire," he told reporters.
"We're in the process of investigating this to the highest level that we can. We're incorporating all of our resources."
He said that there were four smoke detectors in the building but "none of them" had been operating.
The building is owned by Philadelphia's public housing authority.
Eight people managed to evacuate themselves from the house, Murphy said.
'Most tragic day'
The fire department said its officers arrived at the scene in the Fairmount area at 6:40 am (1140 GMT) "and found heavy fire coming from the second floor of a three-story rowhouse."
"PFD responded to a fatal fire this morning on the 800 block of N. 23rd St," it said in a tweet.
"It took 50 minutes to place the fire under control."
Philadelphia mayor Jim Kenney told reporters that "this is without a doubt one of the most tragic days in our city's history, the loss of so many people in such a tragic way."
The Philadelphia Inquirer quoted police as saying that the three-story home had been converted into two apartments.
About 26 people had been living in the building, eight on the first floor and 18 across the second and third floors.
It wasn't clear how many people were in the building during the fire.
"Obviously the tragedy happened and we all mourn for it. But we can't make judgment on the number of people in the house," added Kenney.
Neighbors reached by local media said they had been shaken by the fire.
Bill Richards, who lives nearby, told the Inquirer that shortly before 7:00 am he had heard a woman scream "Oh my god, oh my god."
"It's totally devastating and upsetting," he said.