The discovery was made shortly before 10:00 a.m. in the 1300 block of Dickinson Street.
Police say the killings were so violent that it had to be an act of rage by someone who knew the victims.
"Obviously, that's what we're thinking, but we'll be searching all different avenues, but it was a very violent attack," Philadelphia Police Captain James Clark said.
The victims are a white man in his late 60s and a black woman in her 30s.
The female victim was found in a front room with a belt around her neck. The male victim was found in a rear bathroom with a phone cord around his neck.
Both had their throats slashed and they were stabbed multiple times.
Investigators found the handle of the knife, but not the blade.
The female victim had also been sexually assaulted.
Two dogs belonging to the man who lived in the apartment were also found stabbed to death and there is evidence they were tortured first.
Sources say police found two sets of footprints in the blood-splattered apartment, indicating there may have been more than one attacker.
Police also found drug paraphernalia inside the apartment.
The killers also tried to destroy the scene of the crime by turning on the gas when they left.
Police say the bodies were found when workers responded to the scene looking for the source of the gas fumes.
Hadia Connor lives in one of the other apartments and smelled the gas.
"I saw the police cars and fire department, so I go downstairs and I ask them 'is the gas coming from here' and they say 'yes,'" Connor said.
No one heard any sounds coming from the apartment.
Neighbors knew the elderly man and the younger woman as Gene and Jan and would often bring them food and cigarettes.
They're horrified by what happened here and say they've seen some suspicious characters coming and going.
"It's a nice neighborhood, but we do have this building and a couple other apartment buildings that there's some iffy goings-on," a neighbor named Valerie said.