Investigators are continuing work to identify other suspects in the shooting, officials said.
The mass shooting unfolded around 2 a.m. on the 1200 block of North Alden Street in the Carroll Park neighborhood of West Philadelphia, where a party attended by more than 100 people was taking place, according to police.
When officers arrived, they found victims suffering from gunshot wounds and multiple spent shell casings scattered in the area, officials said.
"It appears that there were multiple shooters. It's unclear at this time how many. But we have counted at least 25 to 30 spent shell casings at this time," Pace told reporters at the scene Sunday morning.
One gun was recovered at the scene, officials said.
"Preliminary information indicates an exchange of gunfire between multiple parties resulting in a nonuple shooting and triple homicide," police said in a statement.
Two of the victims who survived the shooting were taken to hospitals in private vehicles, police said.
The shooting occurred when a fight broke out at the party, police said.
Police say two of the fatally shot victims were brothers.
"My condolences to the family, particularly the parent who lost two sons," Philadelphia Police Commissioner Kevin Bethel said at a news conference on Monday. "It was not some individual who came up and just randomly started to shoot down the block and shoot nine people. There is an absolute gun battle that goes on, for, though it seems long, it was a short period of time."
Surveillance video reviewed by investigators captured the shootout, police said.
"This tragedy reminds us that while gun violence continues to decline in Philly and nationwide, we as elected and community leaders still have much work to do," Philadelphia District Attorney Larry Krasner said in a statement