All seven people wore orange paper wristbands that could provide clues, said Wendell Hissrich, director of public safety in Pittsburgh. He said everyone should be middle-aged men.
The authorities responded to an unconscious man's report at 2 am on Sunday on the south side of the city. They found a man dead at the scene, Hissrich said.
Shortly thereafter, paramedics responded to another man who was "somewhat conscious" and had symptoms similar to those of the first man. According to Hissrich, he was taken to a nearby hospital.
The police took both men back to an apartment where five other people were found. CNN subsidiary KDKA identified the complex as Southside Works City Apartments.
Two of the five people were found dead and the other three were hospitalized.
According to Hissrich, hospital admission conditions range from severe to critical.
The owners of the apartment building were cooperative and, according to Hissrich, there are no indications of security concerns in the building.
Police investigate venues where wristbands have been distributed.