On May 2, 2019, International Isotopes, Inc. (INIS), a subcontractor to Triad National Security, LLC (contractor for Los Alamos National Laboratory) breached a sealed radioactive source containing ~2800 Ci of the radioactive isotope cesium-137 (137Cs) at the University of Washington Harborview Research and Training (HRT) Building in downtown Seattle while attempting to recover the source for the National Nuclear Security Administration’s (NNSA’s) Off-site Source Recovery Program (OSRP). The breach of the source resulted in the contamination of 13 personnel, all 7 floors of the building, and a release of material to the environment. A summary of root and contributing causes of the event was investigated and published by a joint investigation team (JIT) co-led by NNSA and Triad National Security, LLC. That investigation was supported by forensics examinations provided by Pacific Northwest National Laboratory. This report summarizes the results of those forensic examinations.