A man was killed and at least seven other people, including four teenagers, were injured when gunshots rang out at a graduation party in Philadelphia on Sunday night, police said.
No one was in police custody and the police, frustrated after a particularly violent weekend, said they feared reprisals.
Four adults, all in their 20s, were shot and wounded shortly after 10 pm, police chief Richard Ross told reporters around midnight. One of them, a man, was declared dead thereafter.
Four miners between the ages of 15 and 17 were shot, mostly on the legs, Ross said.
The shooting occurred as about 60 people attended a graduation ceremony at an address near two playgrounds in southwest Philadelphia, police said.
Ross stated that there may have been more than one shooter, that it was not known whether the shooter or shooters had escaped into a vehicle or on foot and that the officers were on the lookout for a potential retaliatory attack.
No other information was immediately available.
The shooting took place at the end of a long weekend of violence in Philadelphia, during which at least three other people were killed and 15 others wounded throughout the city.
Ross said the police were preparing patrols as an answer, but he added that the police could only do a lot.
"These are the things that make us absolutely crazy," Ross said, referring to the police department.
"If you just keep asking us, we will never be able to solve this problem," he said. "What you need to know is the hearts and minds of people who want to pull out a gun and shoot at a group of 60 people.
"It's something that is even more disturbing – that you can do that by realizing the carnage you could cause," he said.