It happened just after 7 p.m. on the 3500 block of Fairmount Avenue in the city's Mantua neighborhood.
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Police say the injured victims include a 19-year-old man, a 19-year-old woman, her 34-year-old mother and a 60-year-old man.
The 19-year-old man remains in critical condition at an area hospital. The other three victims are expected to survive their injuries.
According to Philadelphia Chief Inspector Scott Small, the group was heading to a prom party when the gunfire erupted.
"We know at least nine shots were fired from a semi-automatic weapon. We found nine spent shell casings on the sidewalk and some on the front lawn of a property in the 3500 block of Fairmount," said Small.
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Police say surveillance video captured a male gunman who was reportedly on a bike when he fired the gunshots.
The suspect is described as wearing all black clothing and a white mask over his face.
Anyone with information on this case is asked to call police at 215-686-TIPS.