Officers were called shortly after noon Monday to a home in the southwest part of the city, after a relative requested a wellness check.
In the basement of the home, four people - two men and two women, believed to be in their 30s - were found with a gunshot wound to the head each.
"All four of these individuals were executed. There are no ifs, ands or buts about it," Philadelphia Police Commissioner Richard Ross said.
Ross says the home appears to be in the process of being renovated. In fact, Ross says a neighbor heard bangs Sunday night, which he assumed had to do with ongoing work at the house.
There were no signs of a struggle, which leads investigators to believe the victims may have been led down to the basement.
"The house is not in disarray. It doesn't look like there was a struggle upstairs. Just looks like they were either taken down there or they were already down there for some reason and they were shot. It doesn't appear to be a struggle," Ross said.
The neighborhood is rapidly gentrifying, and the block is considered safe. This level of violence stunned neighbors and even the commissioner, who went to see the crime scene for himself.
"I can't get my mind around the fact that you have people walking around, capable of such vicious acts," Ross said.
Police have a tentative identification on at least two of the victims. They believe at least one of the men lived in the house, possibly with the other, who may be his cousin. Police are unsure of their relationship to the women found dead in the home.
So far, no arrests have been made.
Police say they are currently checking surveillance cameras in the area, as well as interviewing witnesses and family members in an attempt to figure out what may have led to the killings.