A boy and a girl hanging out with a group of fellow students in a high school gym were shot and wounded Friday afternoon, police said.
One boy originally thought to have been involved in the shooting was captured near his home but was questioned and cleared Friday night, police said.
A second boy turned himself in with his family, and a third was being sought.
The shooting happened at the Delaware Valley Charter High School in north Philadelphia around 3 pm, authorities said.
Surveillance video shows a group of about seven students hanging around in the gym when the shooting happened, police Commissioner Charles Ramsey said. It was unclear if the shooting was accidental or intentional, but it "doesn't look like an active shooter type of situation," he said.
The wounded boy and girl each were shot in an arm, but it was unclear whether one or two shots were fired, Ramsey said.
The wounded students, both 15-years old, were taken to a hospital near the school, police said. The girl was discharged Friday night, and the boy was listed in stable condition.
The gun used in the shooting hadn't been recovered, Ramsey said.
"We have not found a gun, but there's two, perhaps three, youngsters that were together," Ramsey said. "We're not sure which one fired."
Ramsey didn't know what staffing or security procedures were in place at the school but said "there was a breakdown in security of some kind" because the gun got into the building.
Former student Donte Jenkins, who graduated last year, said there were metal detectors at the entrance and he couldn't figure out how someone could have gotten in with a gun.
"It was always a safe environment," Jenkins said. "It was never a bad environment. I don't know how this really happened."
The school was placed on lockdown and secured by SWAT officers checking each room. Students were searched and released one by one.
Terry Starks, an outreach supervisor for the gun control group Philadelphia CeaseFire, was outside the school Friday evening and was trying to contact his son, a student.
"I'm a gunshot victim so I already know how being shot affects the family," he said. "I feel bad for the mothers and the fathers of these children that got shot. ... Your heart drops."
A spokesman for the school district didn't immediately respond to a call seeking comment.
The president of the charter school's board of trustees said he didn't have any additional information about the shooting, not the only U.S. school shooting this week.
Authorities said a seventh-grader opened fire Tuesday in a New Mexico middle school gym, wounding two classmates.