Group Violence Interventions (GVIs) combine a focused deterrence law enforcement approach with community mobilization and social services. The current study qualitatively examines Project Longevity, Connecticut's largest GVI initiative, to contribute to the limited literature on implementation of gun violence reduction strategies. Relying on interviews with 24 of Project Longevity law enforcement and non-law enforcement partners, we explore the establishment of interagency collaboration, which was viewed by study participants as the most pressing implementation challenge of Project Longevity. Our case study results offer important lessons to practitioners responsible for implementing GVI strategies.