"From what they're telling me, somebody set off a firecracker, and that it's what kind of started all this," Chester Mayor Wendell Butler said. "It just kind of went bad from there."
Authorities have arrested one alleged shooter and charged him with murder, attempted murder, and other crimes, Butler said, and the investigation was continuing.
"I wouldn't be a bit surprised if they were going after other persons of interest," he said.
Chester police officials and the Delaware County District Attorney's Office released no information Saturday on the alleged shooter or the victims.
City and community leaders held a vigil at the scene of the shooting Saturday evening, as word spread that the second victim had died. Butler said he did not know the victims' ages, but he said: "Everyone involved was a young person."
Bernadette Thomas, who lives in the neighborhood, spent part of Saturday washing blood from her sidewalk with a garden hose. One of the victims, wounded in the leg, had sat on her porch step before being taken to Crozer-Chester Medical Center.
Minaret Temple is often rented out for parties, but "not like that" one, said Thomas, who has lived on West Fourth Street, lined with well-kept brick homes, for eight years. The neighborhood is blocks from PPL Park, where the Philadelphia Union soccer team plays its home games.
Butler said word of the party apparently spread on Facebook, attracting teenagers from Philadelphia and Salem, N.J. He said that black-clad men were there to work security but that he did not know whether they were from a licensed company.
He said an adult had rented the venue, but "I don't even know if that gentleman was on scene" when the shooting started.
"There was an awful lot of young people there," he said.
One victim remained in critical condition, and six were listed in stable condition at Crozer-Chester Medical Center. Two people were treated and released, a hospital spokesman said.
One of the gunshot victims was identified by a relative as Breon Bruton, a junior at Chester High School. According to sports websites, Bruton is a running back on Chester's football team and runs track.
Lynda Jacobs of Wilmington, Bruton's aunt, said her nephew had been shot in the leg and was expected to recover.
"He's a good kid; he was just in the wrong place at the wrong time," she said as she arrived at Crozer-Chester to visit Bruton.
Inside the hospital, Patreesha Green and Kani Ramos, both 16, were having a drink in the cafeteria after learning that they were not allowed to visit a friend who also had been shot in the leg at the party. The friend's mother was forbidding additional visitors, the girls said.
The party was a "regular party," not a birthday party or anything special, they said. Both had missed it.
Green said she had to work. Ramos said she "was supposed to go, but I fell asleep."
The two said they did not know what led to the gunfire, but they said that people from different neighborhoods mix at large parties such as that one. It ended with teenagers stampeding out onto West Fourth Street.
Above the whine of weed trimmers, neighbors gathered on covered porches Saturday to talk about the shootings. Among them was Lawrence Wilson, who said his nephew was at the party, which was thrown by what Wilson called a "well-known teenager" who attracted a large crowd.
Residents Johnson and Thomas said they would like to see Butler bring back the state of emergency and a 9 p.m. curfew that was imposed in problem areas last June after a rash of shootings left four people dead in eight days.
Thomas said she was fed up with the violence: "It's just got to stop. It's got to stop."
Contact staff writer Harold Brubaker at 215-854-4651 or firstname.lastname@example.org.