The investigation comes as authorities continue to examine several other recent incidents involving children.
Police said officers were called on the afternoon of Dec. 14 to the 3800 block of Fairmount Avenue for a report of a choking child. The baby girl was taken to a hospital, where she was pronounced dead.
Neighbor John Young said the police presence quickly grew.
"Some more cops came, and some more cops came, and they were out there for a long time," Young said.
Police tell Action News that following a review by the medical examiner, the circumstances surrounding the baby's death warranted further investigation.
The case was transferred from the Special Victims Unit to the Homicide Unit. The autopsy report, including the cause and manner of death, has not yet been released.
Young said the extended police activity made him suspect something more serious had occurred.
"I figured something else - something happened for them to be there that long. And even into the next day," he said.
The case comes as police continue to investigate two recent violent incidents involving children. Authorities say 2-year-old Key'Monnie Bean was beaten to death in South Philadelphia on Dec. 8. Her mother's boyfriend, Sean Hernandez, was charged with homicide last week.
In another case, police say a mother and her 5-month-old baby were shot inside a West Philadelphia home over the weekend. Both are expected to survive. The victim's sister told police the shooter is the baby's father.
David Chiles, executive director of the Lutheran Settlement House, said the organization often sees an increase in domestic violence cases during the holiday season.
"While it wasn't surprising to see an incident, it's heartbreaking," Chiles said.
"There's a lot of dynamics in relationships, and any time you bring a lot of stress into those dynamics, unfortunately, if there's already abuse present, it can bring it up," he said.
Chiles encouraged residents to be aware of Philadelphia's 24/7 domestic violence hotline. The number is 1-866-723-3014.
Police said investigations into all three incidents remain ongoing.
In the shooting involving the mother and infant, detectives are still searching for 39-year-old Faheem Weaver, who they believe is driving a silver Toyota pickup truck.