Police were called around 12:45 p.m. to the Westpark Apartments, a Philadelphia Housing Authority property, on the 300 block of N Busti Street for the report of a child that fell out of a window.
Witnesses saw the bleeding child on the pavement below as his mother came running and screaming for help from the building.
"She ran past everybody. She was like 'my baby, my baby,'" said Starr Glenn of Powelton.
Detectives were called to the scene as 18-month-old Hezekiah Robinson was taken to Children's Hospital of Philadelphia.
Hezekiah ultimately succumbed to his injuries just before 3:30 p.m.
The boy's mother, Lonnia Robinson, told investigators she had been cleaning in another room.
Hezekiah and his 3-year-old sister were playing alone in the bedroom when he fell out of the open window.
The window had no bars.
Police say Lonnia Robinson had removed the air conditioning unit. Her boyfriend says she had filed a work order with the Philadelphia Housing Authority to replace the parts.
"She was not a neglectful parent," said Vincent Payne. "They failed to put up a secure screen for her. I explained that to the maintenance man."
PHA says it's checking to see if a work order was ever filed.
The agency's president and C.E.O., Kelvin Jeremiah, released a statement saying that he's deeply saddened by this tragedy: "The surviving family members have experienced an unspeakable loss, and our entire community must support them during this very difficult time. I ask all the citizens of Philadelphia, especially neighbors in the Westpark community, to keep the family in their thoughts. "
Authorities are calling this incident a sudden death investigation.
No charges have been filed.