A man was killed and at least five other people, four of them teenagers, were injured when gunfire rang out at a graduation party in Philadelphia on Sunday night, police said.
No one was in custody, and police, who were frustrated after an especially violent weekend, said they were concerned about possible retaliation.
Earlier, police had said that seven other people were injured in the shooting that took place shortly after 10 p.m.
The man killed was aged 24, while four teens between the ages of 15 and 17 suffered non-life-threatening gunshot wounds, primarily in their legs, Police Commissioner Richard Ross told reporters around midnight. A 21-year-old was also injured.
The shooting occurred as about 60 people were at a graduation celebration at an address near two playgrounds in southwest Philadelphia, police said.
Ross said that there may have been more than one shooter, that it wasn't known whether the shooter or shooters fled in a vehicle or on foot and that officers were on the lookout for a potential retaliatory attack.
Police found multiple private surveillance cameras in the area of the shooting and they are now working to retrieve the video.
The shooting came at the end of a long weekend of gun violence in Philadelphia, during which at least 27 people were shot.
Ross said police were ramping up patrols in response to the shooting, but he said there was only so much the police could do.
"These are the things that absolutely drive us crazy," Ross said, referring to the police department.
"If you only continue to ask us, we're never going to solve this issue," he said. "What you've got to get at is the hearts and minds of people who want to pull out a gun and fire at a group of 60 people.
"That's something that's even more troubling — that you could do that realizing the carnage that you could cause," he said.