This ongoing field study was conducted subsequent to the discovery of clandestine graves with 218 bodies recovered in the Mexican state of Durango in April 2011. A preliminary psychometric assessment was conducted with the 60 State Attorney General employees who were working with the corpses to establish a triage criterion and provide baseline measures. The Impact of Event Scale (IES) and the short posttraumatic stress disorder (PTSD) rating interview were administered, and the 32 individuals whose scores indicated moderate-to-severe posttraumatic stress and PTSD symptoms were treated with the eye movement desensitization and reprocessing (EMDR) Protocol for Recent Critical Incidents (EMDR-PRECI). Participants were assigned to two groups: immediate treatment (severe scores) and waitlist/delayed treatment (moderate scores). Each individual client session lasted between 90 and 120 minutes. Results showed that one session of EMDR-PRECI produced significant improvement on self-report measures of posttraumatic stress and PTSD symptoms for both the immediate treatment and waitlist/delayed treatment groups. This study provides preliminary evidence in support of the protocol’s efficacy in a natural setting of a human massacre situation to a group of traumatized adults working under extreme stressors. More controlled research is recommended to evaluate further the protocol’s efficacy.