One person has been arrested in connection with the mass shooting that left three people dead and nine injured on a residential block in Grays Ferry last month, Philadelphia police said Thursday.

Terrell Frazier, 22, is charged with three counts of murder, nine counts of attempted murder and several other offenses connected to the July 7 shooting. Police are still looking for at least 12 more suspects.

The deadly shooting happened just before 1 a.m. July 7 on the 1500 block of South Etting Street. Police soon released Ring camera surveillance images of three suspects who were wanted in connection with the shooting, which involved several different shooters.

Late last month, police released more photos of the suspects.

Over 140 pieces of ballistic evidence were found at the scene, and investigators have traced the bullets to 13 different guns.

"I was on that block within an hour or so of this incident, and I think every parked car had a bullet in it," Philadelphia Police Deputy Commissioner Frank Vanore said in a news conference.

The shooting likely stemmed from one person firing a gun at a gathering of about 100-200 people on the block, Vanore said. There was no fight or obvious target, police said.

"We believe that one individual that was amongst their group had fired a gun. We believe it may have been in the air," Vanore said. "That caused at least a dozen other people with firearms to then begin recklessly and really intentionally firing their firearms. I have no idea what all of them were firing at, but they fired multiple times, and that's what caused three people to be killed and nine people to have significant injuries from gunfire in this incident."

Prior to the shooting, officers had already been on the block on the Saturday leading up to the shooting and made multiple arrests. Police then returned the following day after a report of loud music.

The day of the shooting, police officers were nearby responding to a different incident when they heard gunfire and rushed to the block.

Vanore said investigators are close to charging other people in this shooting.

"We have some good leads, but we still need help with this case," Vanore said, referring to the previously released Ring camera images. "We need people, if you know who they are, to call us."

Police are offering a $20,000 award for any information that leads to a conviction.