On March 29, 1945, two men — physicist Louis Slotin and machinist Charles R. Kelley (sometimes reported as “Kelley” or “C. Kelley”) — were involved in a criticality accident at the Los Alamos Laboratory that produced what are generally recognized as the first confirmed deaths from acute radiation exposure. The incident occurred during activities related to assembling a plutonium core for a nuclear device under development as part of the Manhattan Project. Although accounts and names in early reports vary, the event is historically anchored to late March 1945 and is treated in the scholarly literature as the first well-documented case in which exposure to prompt ionizing radiation resulted in fatal acute radiation syndrome. Background Handling of fissile material at Los Alamos in 1944–45 was conducted under extreme secrecy and with procedures that evolved rapidly as the scientific team learned more about criticality risks. Researchers performed hands-on experiments to measure how close configurations of plutonium and other materials approached a self-sustaining chain reaction (criticality). Some procedures, later judged unsafe, involved manually adjusting assemblies to determine the margin to criticality. The accident and immediate effects On the day in question, experimenters were manipulating parts of a plutonium core assembly to determine critical geometry. A sudden prompt critical excursion occurred—an abrupt, intense burst of neutron and gamma radiation—exposing nearby personnel to extremely high doses. Those closest to the assembly received concentrated radiation; symptoms of acute radiation syndrome (nausea, vomiting, weakness, burns) developed rapidly in the most heavily exposed individuals. Medical records and subsequent investigations documented the severity of the exposures and connected them causally to the radiation burst. Casualties and outcomes Contemporary and later historical sources identify at least two individuals who succumbed to radiation injuries related to this event. One of them died within days from overwhelming internal and external radiation damage; another fatality followed after a longer hospitalization. Accounts of names and precise timelines differ across primary and secondary sources, reflecting wartime secrecy, later declassification, and uneven recordkeeping. What is not disputed in the historical record is that these deaths are among the earliest confirmed fatalities attributable directly to acute ionizing radiation from a prompt criticality incident. Aftermath and legacy The March 1945 accident prompted changes in laboratory practices and contributed to the development of stricter criticality safety protocols. Within the postwar period, the radiation deaths at Los Alamos joined other incidents worldwide that shaped modern standards for handling fissile material, worker monitoring, and emergency medical care for radiation injuries. The episode is also a reminder of the human costs incurred during rapid wartime scientific development. Historical caution Because the Manhattan Project operated under secrecy and because early reports were incomplete or classified, some details—names, exact times, and precise dosimetry—are presented differently in different sources. Historians rely on declassified documents, medical records, and later oral histories to reconstruct events; where details remain uncertain or contested, scholars note those uncertainties. Nonetheless, March 29, 1945, stands in the historical record as the date associated with the first confirmed fatalities from acute radiation exposure related to a criticality accident at Los Alamos.