Those sources also said a weapon has been recovered after gunfire that left eight Northeast High School students wounded.
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Police have not released any more information about those in custody.
Philadelphia officials, including the mayor, police commissioner and the D.A., will hold a 4:30 p.m. EST news conference with the latest on the investigation. They will be joined by officials from the FBI, the ATF and the U.S. Marshals.
The shooting happened about 3 p.m. on Wednesday.
The injured students range between the ages of 15 and 17. One of the victims, a 16-year-old boy, remains in critical condition after being shot nine times. Police say he was the intended target.
Surveillance video released Wednesday night shows three shooters exit a blue Hyundai and open fire. Police urged tipsters to help identify the three shooters and the car's driver.
VIDEO: Suspects wanted in Philly mass shooting that injured 8 at bus stop
VIDEO: Suspects wanted in Philly mass shooting that injured 8 at bus stop
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That evening, police found two vehicles they believe are connected to the shooters, including the getaway car.
Both had been stolen.
Police have said all along they are working to see if the mass shooting is connected to a deadly shooting of an Imhotep Charter High School student, 17-year-old Dayemen Taylor.
Taylor was shot a week ago Monday while waiting at a SEPTA bus stop. Two other students and two bus passengers were also injured.
Teen killed in SEPTA bus stop shooting identified as Imhotep Institute Charter High School student
Students return to class after shootings
Northeast High School students returned to class Monday for the first time since the shooting.
The school switched to virtual learning late last week in the wake of Wednesday afternoon's violence.
Imhotep students also returned to attend in-person classes on Monday. It was their first time back in-person since the killing of their classmate last Monday.
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RELATED: Philadelphia mass shooting victim recalls fleeing gunfire: 'Everything happened so fast'
"I'm not shocked because things like this have happened in the past. The only thing that really shocked me is that I happen to be a victim of it," said one of the victims, who did not want to be identified.
He said he was waiting for his bus ride home on Wednesday when he heard more than 30 gunshots. And without hesitation, he ran in the opposite direction.
He was shot in the upper back.
"It's like a scene out of a movie, an action movie. Everything happened so fast. You have to be on your toes consistently if you want to make it out of a situation like that," he said. "As I was running, I felt a burning sensation in my back, but I ignored it because of adrenaline. But as I stopped, it just felt like I might've got shot."
Anyone with information on the suspects is asked to call police at 215-686-TIPS.