Post-conflict transitional justice has increasingly relied upon the use of conflict-specific ad hoc investigatory bodies to handle fact-gathering for the trials of the alleged perpetrators. However, the process of obtaining evidence through investigative interviews without adequate mental health considerations puts witnesses and victims at risk of retraumatization. In many instances, investigators are trained and highly skilled at interviewing alleged perpetrators but are not experts in working with witnesses or survivors. The United Nations’ Investigative Team to Promote Accountability for Crimes Committed by Da’esh/ISIL (UNITAD) is currently conducting interviews in Iraq with survivors of Da’esh/ISIL atrocities. UNITAD made the key decision at the outset to develop and use a trauma-informed interviewing approach to avoid retraumatizing witnesses and victims unintentionally while still eliciting information that could prove useful in a court of law. To date, there has not been a systematic approach to developing and implementing trauma-informed, evidence-based services that address the impact of trauma on the people who have had their human rights violated. This article describes the development and implementation of a trauma-informed investigative approach that could be used with traumatized populations in the aftermath of gross human rights violations. It also reviews the psychological dangers of not using a trauma-informed approach in investigations of mass atrocities, outlines UNITAD’s trauma-informed interview approach, and presents UNITAD’s plans to train future cohorts of investigators in the use of these techniques.