The suspected shooter was identified as a 40-year-old man. A second person was also taken into custody who may have returned fire at the suspect, but police did not know whether there was a connection between the two, Outlaw said.
The chief described the scene as covering an area of two by four blocks and said dozens of shell casings were found.
“There are several scenes out here,” Outlaw said. “We’re canvassing the area to get as much as we can, to identify witnesses, to identify where cameras are and to do everything to figure out the why,” Outlaw said.
Three of the dead ranged in age from 20 to 59, while the fourth, who had not yet been identified, was estimated to be between 16 and 21. All were male. Two boys, ages 2 and 13 were in a stable condition in hospital, Outlaw said.
The shooting occurred a day after gunfire erupted at a holiday weekend block party in Baltimore, about 160km southwest, killing two people and wounding 28 others. The injured ranged in age from 13 to 32, with more than half of them minors, according to officials.
The Philadelphia violence marks the country’s 29th mass killing this year, according to a database maintained by the Associated Press and USA Today in partnership with Northeastern University.
So far this year, the nation has witnessed the highest number on record of mass killings and deaths to this point in a single year.
More than 550 mass killings in the US since 2006 have killed at least 2900 people injured at least 2000 people, according to the database.