Police placed the girl in the back of a squad car and raced her to nearby Children's Hospital of Philadelphia, only for her to be pronounced dead there within minutes, at 10:29, Walker said.
No charges were immediately filed, and it was unclear who, other than the four children, was home at the time of the shooting, Walker said. All four siblings appeared to have been in the second-floor bedroom when the gun, a .357-caliber handgun now in police custody, went off in the 2-year-old's hands.
Walker said police still were trying to ascertain which of the siblings - who ranged in age from 2 to 14 years old - had found the weapon, carried it up the stairs, and taken it into the room where the 11-year-old was playing on a sunny start to a spring weekend.
The children were "distraught," the lieutenant said.
Walker said police were trying to respect the mother's grief while also questioning her.
"Mom is extremely distraught," he said. "These are just horrible situations."
Walker said that police were "not even at that point" of considering criminal charges and that investigators would consult with the District Attorney's Office.
He said detectives would conduct a fresh examination of the weapon - a handgun with an 8-inch barrel - and work their way back from there, he said. Police were investigating whether the weapon was legally registered and if so, to whom.
"The mom is in a really tough state of mind," Walker said. She called 911, but Walker said he did not know if she was inside when the gun was fired.
The dead child, whose identity was not being disclosed, had lived with her family for years in the house in the city's Mantua section, near the campuses of Drexel University and the University of Pennsylvania, said one neighbor, Alice Freeman, who said she had lived on the 3800 block of Wallace for more than two decades.
Some time during the morning, according to police, the child's mother was home and welcomed a visitor - a male friend of the family's.
"A gun was placed on top of a refrigerator inside of the house . . . by this male," Walker said. When the man left, the gun was left behind.
On Saturday morning, one of the children found the handgun and took it up the steps to a second-floor bedroom.
"It's a tragedy. It's a sad thing," said Maxine Seward, 59, a neighbor. "I don't know what to say. It's just a sad thing. A child is gone."
Inquirer staff writer Andrew Maykuth contributed to this article.