(CNN) Philadelphia officials updated the death toll in Wednesday morning's fire at a three-story row home that was converted into two apartments.
"Fire officials now confirm that 12 people — eight children and four adults — died in the fire on N. 23rd Street. That total is one less than the number reported in an earlier news conference, when recovery operations were still ongoing," an updated news release from the city said.
Philadelphia Deputy Fire Commissioner Craig Murphy initially said 13 people were killed; two others were taken to hospitals; and eight people were able to escape by themselves.
Firefighters faced heavy smoke, heat and limited visibility on all floors when they entered the building, according to the release. At least one child firefighters initially rescued did not survive, the department said.
The fire took place at a home that records show is owned by the Philadelphia Housing Authority, a municipal agency that leases homes to people with low income.
"This is without a doubt one of the most tragic days in our city's history -- loss of so many people in such a tragic way," Mayor Jim Kenney said at a news conference late Wednesday morning.
"Losing so many kids is just devastating. ... Keep these babies in your prayers," Kenney said.
Firefighters responded to flames around 6:40 a.m. Wednesday at the row house at 869 N. 23rd Street in the city's Fairmount neighborhood.
Live updates: At least 12 dead in fire at Philadelphia row home
Firefighters found "heavy fire" in a kitchen area in front of the second floor, and there was "nothing slowing that fire from moving," Murphy said.
"It was terrible," Murphy said. "This is probably one of the worst fires I ever been to."
Twenty-six people lived in the three-story building -- eight on the first floor, and 18 on the second and third floors, fire officials said.
The housing authority was not aware 26 people were living in the building, said Dinesh Indala, the agency's senior executive vice president of operations. The agency is checking how many were allowed to live there, he said.
"You don't know the circumstances of each and every family, and maybe there were relatives and family that needed to be sheltered," Kenney told CNN affiliate WPVI. "Obviously, the tragedy happened, and we all mourn for it. But we can't make judgment on the number of people living in the house because sometimes people just need to be indoors."
The cause of the fire will be investigated, Murphy said.
Philadelphia's district attorney said the city "owes it to the victims, the survivors, and to all Philadelphians to conduct a thorough investigation into this travesty, so that we can make sure it never happens again."
"I join Philadelphians in expressing my heartbreak over the tragic loss of life in Fairmount today," District Attorney Larry Krasner said.
Pennsylvania Gov. Tom Wolf tweeted he was devastated.
"My heart goes out to the loved ones left to cope with this heartbreaking loss of life. Thank you to the brave first responders who got the fire under control," Wolf's tweet read.
First Lady Jill Biden also tweeted Wednesday, saying her heart was with the families and loved ones of the victims.
Neighbors and others -- some sobbing -- gathered outside the burned row house as firefighters and police worked the scene Wednesday morning, WPVI reported.
Lindsay Hull told WPVI that she saw stretchers when she came up to the site at 7 a.m. A friend lives next door to the row house, she said.
"That's a house that has a lot of kids," Hull told WPVI. "It's sad."
Bill Richards, who said he's lived on the block for 24 years, told WPVI that before he knew of the fire, he heard a woman yell, "Oh, my God! Oh, my God!" He then heard fire trucks and went outside.
"It's very upsetting," Richards told WPVI. "I just can't wrap myself around it."
Richards told CNN he was a teacher his entire career, and he can't wrap his mind "around the tragedy of these relatives that are going to have to pick up their lives after this."
"And I taught at the school down the street, a block away and some of those kids went there," he said.
Richards described the area as "a very family-oriented neighborhood."
"We'll help each other get through the grief," he said.
Murphy initially told reporters that four smoke detectors were in the building, "and none of them operated."
Murphy later indicated that Philadelphia Housing Authority records showed that at least six battery-operated smoke detectors had been installed there from 2019 to 2020.
However, Indala, the housing authority official, said the agency had different information about the detectors.
One of the apartments, the "A unit," had seven smoke detectors and three carbon monoxide detectors at its last inspection, Indala said. He did not specify the year of the inspection, and CNN has requested clarification.
The other apartment, the "B unit," had six functional smoke detectors and three functional carbon monoxide detectors as of its last inspection in May 2021, Indala said.
Two batteries and two smoke detectors were replaced then, Indala said. Smoke detectors also were replaced in the B unit in an inspection in September 2019, according to Indala.
When a reporter asked why smoke detectors would not have worked if they were inspected in May 2021, Indala replied, "I don't know if they were replaced or tampered with. ... We are working with the fire department at this time to do further inspections."
It wasn't immediately clear which floors the A and B units covered.
Faulty smoke detectors are treated as emergencies and are replaced in 24 hours if requested, and the authority does inspections annually, Indala said.
"Every time we come in for an inspection, as is evident from the last one, we had to replace two batteries, replace the smoke detectors. And these are 10-year smoke detectors, so that's something we run into quite often on our properties," Indala said.
The row home has been legally subdivided into two apartments since the 1950s and has had no violations, according to a spokesperson for the Philadelphia Department of Licenses and Inspections.
The home has had three minor complaints since 2019, all related to trash maintenance, department spokesperson Karen Guss said.
One complaint was about trash on the exterior of the property in September of 2021 and October 2021. A complaint about trash and poorly maintained interior surfaces was made in 2019, Guss said.
The building, according to records, was estimated to have been built in 1920.
The home is about 2.5 miles northwest of Philadelphia's Center City district.