A fire that burned through a row house early Wednesday in Philadelphia killed at least 13 people, including seven children, authorities said. Fire officials warned that the numbers could increase because 26 people had been staying in the house owned by the city's public housing authority.

The cause of the fire remains under investigation, and the number of fatalities could increase because firefighters were still searching the building, Philadelphia Fire Department Deputy Commissioner Craig Murphy said. The death toll is the highest in a single fire in the city in at least a century, The Associated Press reported.

Two people, including a child, were taken to area hospitals and listed in critical but stable condition. Eight people were able to escape the burning duplex, Murphy said.

At least four battery-operated smoke detectors in the building with 10-year lithium batteries didn't go off during the fire, he said.

Fire officials said there were smoke-detector inspections and installations at the house in 2019 and 2020. Officials with the Philadelphia Housing Authority (PHA), owners of the property, said that all smoke detectors were working when they last inspected it in May.

The building had been legally subdivided into two apartments since the 1950s and has had no violations, according to the Philadelphia Department of Licenses and Inspections.

"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," Mayor Jim Kenney said at a news conference. "Losing so many kids is just devastating."

The fire on the second floor of the building in the city's Fairmount neighborhood was first reported at about 6:40 am as flames shot from front windows in an area believed to be a kitchen, Murphy said. It took 50 minutes to get the fire under control, the fire department said.

At least 18 people were living in the upstairs apartment, and eight people were living in the downstairs unit, which included the first floor and the other part of the second, Murphy said.

The deputy fire commissioner said he couldn't say whether there were more people in the house than what would be allowed but called it a "tremendous amount of people to be living in a duplex".

PHA officials said they weren't aware that 26 people were living in the building, and they are working to determine how many people were allowed to live there. However, they acknowledged 26 was likely too high.

Regina Cureton, 48, a nearby resident whose building has a layout similar to the property, said she raised concerns about fire escapes two years ago when she moved in. She said the only exits are the front and back doors.

"[My family and I] were afraid that this was going to happen, and they never took care of that issue," she told The Philadelphia Inquirer.

Cureton said she worries that even with working alarms in her home, she and her children would have no way of escaping if the main exits were blocked.

Murphy said that the fire was "not necessarily considered suspicious" but that the investigation would be "all hands on deck. We plan on making sure that this tremendous loss of life did not happen in vain," he said.