Aaron Eugene Wallace, 21, was arrested Friday in Lancaster in connection with the slaying of 14-year-old Dominique Peatry.
In early September 2010, Peatry was at a birthday party when an argument broke out. Gang members turned up and began shooting.
Peatry, who was killed, was one of six adults and children shot in the incident.
Authorities said the public helped them identify Wallace as they continued their search for a second suspect.