The mother of the two boys, Elenor Jacque, 21, was not at the house when the fire broke out.
It took 100 firefighters 11/2 hours to control the fire, which initially came in as a rubbish-fire call about 2:40 a.m., Fire Commissioner Derrick Sawyer said. The firefighters from the station just around the corner arrived in three minutes, he said.
"The members of that station are heartbroken now," Sawyer said. "They tried to keep the fire to four homes, but at some point the fire extended to eight homes."
Neighbors in the mostly Liberian community speculated that the fire might have been started by youths playing with leftover Fourth of July fireworks.
"I've heard the same rumor, but we're not going to speculate," Mayor Nutter said, adding that the Fire Marshal's Office was investigating. "They will figure out source, cause, and origin of a fire."
Nutter said there was "no indication" of an accelerant.
"It was a little bit windy yesterday," Sawyer said. "Once the fire gets on the porches it spreads quickly."
Heat from the blaze melted plastic on parked cars and homes across the street.
The house where the children died had two new smoke detectors. It was one of several homes on the block that firefighters visited last summer in a fire-safety outreach effort.
Family and friends of those affected by the fire hugged one another Saturday and shook their heads at the horror of it all. Some gathered to pray at the Christian International Baptist Church on the corner, and in the middle of the one-way street.
Anton Moore, who runs Unity in the Community, an outreach group against gun violence, led one of the prayers.
"Please bring peace and unity to the families," Moore said as he held hands with about 40 people. "We ask that the community come together as one."
Moore was planning to hold a fund-raiser for the families on the block. Several women held an impromptu food and clothes drive at the end of the street.
"When something happens in the community, you have to be there for your sisters," Sarah Sakie said as she restocked a cooler with bottled water.
Jeff Boone, 27, who lives at 6526 Gesner, said he was playing games on his PlayStation while his niece slept upstairs when he stepped outside and noticed flames about five doors down.
"As I saw the couch on fire, I ran back," he said, "and grabbed my fire extinguisher."
But it didn't work.
He said the flames spread from porch to porch so quickly "it looked like someone had a flamethrower and just shot it all across."
Boone called 911. He then ran around, trying to get people out.
"I was running, screaming, telling everybody, 'Get up! Yo, get, yo! Go!' "
He said he ran around the corner to the fire station and saw the engine about to leave.
Boone said there was not much he could do after that.
"The flames were just so intense, I couldn't see anything from the smoke," he said. "I heard the little kids upstairs screaming."
The four victims were found in a second-floor front bedroom of the house in the middle of the one-block-long street in the Mount Moriah section.
Bowah and her three surviving daughters suffered non-life-threatening injuries while making their escape.
Jaka Kaba's house also was damaged by the fire. But she was distraught over the deaths of the children.
"I knew them. Everyone knew them. They were good children," Kaba said. "They didn't deserve this. No one expects this. It's hurting everyone."
Milton Musa, whose house at 6516 Gesner was caught in the fire, was awakened by his alarmed roommate.
" 'Fire, Fire!' he yelled," recalled a shaken Musa. "So I got up and ran out of the house as fast as I could."
Once out of his house, Musa noticed two children hanging from a neighbor's window and stopped to help carry them to safety.
"I could see they weren't strong, and I was afraid they'd fall to the cement," Musa said. "So I went underneath them, let them fall on my back, and carried them away."
"Everyone was running for their lives," Musa said as he wiped away tears. "I've lost everything. My paperwork, my documents, my house. Everything."
The fire displaced at least 30 residents, from 6510 to 6526 Gesner.
"We know there will be more," Renee Cardwell Hughes, chief executive officer of the American Red Cross of Southeastern Pennsylvania, said. "These are multigenerational homes."
Affected residents returned to the scene Saturday afternoon.
"All of it is gone, baby. All of it is gone," Teenamarie Shaw said as she sobbed on the shoulder of her husband, Maurice. The couple have been living at 6514 for four years.
Crews from the city's Department of Licenses and Inspections will be tearing down the porch roofs and sealing the burned-out homes to prevent people from entering. Eight of the homes were deemed structurally unsafe.
There were 24 fire deaths in Philadelphia last year, including one child, the fire commissioner said. Still, Saturday's deaths of four children in a single blaze were not the most in the city's history.
In 2005, an early-morning rowhouse fire in Kensington took the lives of five children between the ages of 1 and 5.
In 1967, a Fairmount rowhouse fire killed a couple and seven of their 12 children. In the 1980s, two separate fires killed five and six children.
"We lost four precious lives," Nutter said Saturday afternoon. "Four little, innocent children, in a horrific tragedy."
Inquirer staff writers Mike Newall and Joseph A. Gambardello contributed to this article, as did Robert Tornoe of Philly.com.