It happened at 3:18 p.m. Sunday on the 300 block of Stevens Street.
The victims have been identified as 49-year-old Donna Muller and a 22-year-old Richard Muller.
"It's definitely a violent crime scene because you have two dead bodies that appear to be shot in the head," said Chief Inspector Scott Small.
Sources say a family member made the discovery.
Both victims were found shot in the head. One body was discovered in a bedroom and the other in a hallway.
Police believe a semi-automatic gun was used but no weapon was found at the scene.
Detectives also say it appears the victims may have known their killer.
"My friend told me and I didn't believe it, I thought he was lying," said William Fullerton.
Fullerton nearly collapsed outside of his best friend's home after hearing the news.
"When I finally got to the door I realized," said Fullerton.
Neighbors describe hearing noises overnight although it's not yet clear when the shooting actually happened.
"I just heard two pops," said Peter Riley.
"She said she heard a lot of banging and noise at 3:30 this morning," said John Downey.
The victims' extended family, upon learning the news, say they have no idea who would want to harm the Mullers.
"This is unbelievable. I don't know what to say really. Donna was a great woman," said Jim Kunf.
No word on any suspects or a possible motive. Meanwhile, an autopsy is scheduled for this week.
A $20,000 reward is offered for the arrest and conviction of the suspect(s) in any homicide in the City of Philadelphia.