To support broader Army efforts to operationally define squad lethality, our team conducted foundational research to identify team performance measures in Army fire teams engaged in a support by fire task. Based on a review of existing literature and field training observations, we identified critical measures of teamwork and weapon engagement.  These measures were collected from eighteen squads that executed battle drill 2A (squad attack) once during the day and once at night using live fire. Squad communication measures, including information exchange, leadership, communication quality, and implicit coordination, were captured using audio recording devices placed on each member of the support-by-fire team. Lethality measures included probabilities of hits and kills, target suppression, changes in firing rate and distribution of shots around each target using location of miss and hit (LOMAH) sensors. This paper examines the role of team interactions including the type and timing of information shared, leadership and coordination on lethality measures.  The impact of demographic measures like time in service and record fire score are also accounted for.  These findings are being used to identify squad level measures of teamwork that are critical determinants of squad lethality for use in a squad performance model.