Police served a search warrant in the West Philadelphia area on Wednesday and arrested 22-year-old Terrell Frazier.
Along with the murder charges, Frazier is also charged with nine counts of attempted murder and other offenses.
But police are still looking for at least a dozen more shooters after finding 13 separate guns and 140 pieces of ballistic evidence at the scene.
"This arrest is not the end of this investigation," said Deputy Commissioner Frank Vanore, with the Philadelphia Police Department. "There is a lot more work to do."
Police give significant update on Grays Ferry mass shooting
The shooting happened just before 1 a.m. on July 7 in the 1500 block of South Etting Street. Shocking video from the shooting shows multiple people pulling out guns and opening fire.
Police said 13 separate guns were fired in this incident. In the end, three people were dead and nine others were wounded.
Officials said on Thursday that there was no argument or opposing party at the events. They believe there was one individual among the group who fired a gun, possibly into the air, which caused at least a dozen other people with firearms to begin "recklessly and, really, intentionally," firing their guns.
"I don't think I've ever seen contagious gunfire like this," Vanore said.
Vanore said they are very close to arresting a few others and believe other charges are coming.
"We just need time to put all the evidence together and make sure it's reviewed properly," he said.
ALSO SEE: Video shows multiple people opening fire in Grays Ferry mass shooting that left 3 people dead
The shooting occurred during a gathering of roughly 100 to 200 people, police said.
When police arrived on the scene, they found a dozen shooting victims ranging in age from 15 to 24.
Police had previously released images of several suspects seen on Ring video.
The victims who were killed have been identified as 23-year-old Zahir Wylie, 19-year-old Jason Reese, and 24-year-old Azir Harris.
Among the victims who were injured are a 15-year-old girl and two 17-year-old boys.
The other shooting victims range in age between 18 to 24 years old.
The night before the shooting, police said there was a "social media-initiated gathering" on the same block that was allegedly for a birthday party. Officials said roughly 100 to 200 people also gathered in the area for this event, bringing together people from all different sections of the city.
The police were called to disperse the crowd. However, some of the people continued to "act recklessly," including jumping on police vehicles. Two people were arrested for assault on police that day.
Officials said after police broke up the group, the party was reannounced and many met back up on the same block, continuing the gathering into the next day.
Police said they collected a lot of social media evidence from the gathering that took place the night before the shooting, which helped them in the investigation.
Anyone with information on any of the shootings can call or text Philadelphia police at 215-686-TIPS (8477) or submit an anonymous tip online.