Background: On May 6, 2023, an armed gunman opened fire at a shopping mall in Allen, Texas. Eight people including the perpetrator were killed with more injured. Medical City McKinney, a level 2 trauma center received seven patients. Medical City Plano, a level one trauma center received one patient. Medical City Dallas Children’s Hospital, a level three pediatric trauma center received one pediatric patient. Methods: Medical City North Texas Division command center engaged Emergency Department leadership at North Texas facilities to prepare for transfer of patients. Ground transport and an air medic team on standby was staged early on. Nearby non-trauma centers sent available staff for support. Results: Small group debriefs occurred with team leaders, followed by a larger internal debrief. Debriefs were also conducted externally with division leadership, Medical City Dallas Children’s Hospital, Medical City Plano, and the North Texas Transfer Center. The lessons learned have been divided into five categories:  1. Communication:  • Official communication of the MCI plan via an overhead alert and call to leadership. • The development of a succinct MCI Response Plan Checklist. • An emergency contact number for the Medical Examiner. 2. Roles and Responsibilities:  • The command center was established which expedited the labor pool, patient tracking, and the Family Reunification Center.  • An ER RN staged at the EMS bay functioned as the Triage Unit Leader. 3. Mobilization of Resources:  • Mobilization of staff allowed for more extensive care without depleting resources.  • Early allocation of pediatric transportation prevented delays in care.  • Environmental Services was needed to manage turnover of the trauma rooms.  4. Safety, Security and Forensics:  • Forensic training is essential and continued education should be available.  • A set of ID badges should be available for emergent staff access.  5. Clinical Care and Support of the Patient and Staff:  • Recovery of the surgery schedule took several days.  • The behavioral health team provided PTSD and depression care early • Rehabilitation physicians provided early evaluations and facilitated the transition from hospitalization, rehabilitation, and eventually home for patients and their families. Conclusions: Further development of tabletop exercises, mock MCI drills, additional education for staff, frequent review of the MCI Response Plan Checklist, and implementation of additional protocols are anticipated. It is imperative major trauma facilities remain prepared for mass casualty incidents with strong protocols, regular drills, behavioral health and rehabilitation support, and thorough local and regional debriefs.