In the mid-1940s the US was leading the world in atomic weapons development and the understanding of radioactive materials. Just ten years after plutonium had been discovered, the Manhattan Project was already close to creating a working atomic bomb. Such advances had not been made on the safe handling of such materials, however. This lack of understanding led to researchers on the project injecting people, unwittingly, with plutonium, to study the effects.
The Manhattan Project was the name of the major US research and development program that produced working atomic weapons. It began officially began in 1942, but similar, less intensive research had been ongoing since the late 1930s. The US took the idea of atomic weapons seriously in 1939 when President Franklin D. Roosevelt received the Einstein–Szilárd letter from Hungarian physicists. This letter warned the US about the potential German development of atomic weapons and was signed by Albert Einstein.
While it’s most famous for the development of the atomic bomb, the Manhattan Project encompassed a number of different paths of research at different sites.
Much of this was related to the incredibly complex development and production of the weapons themselves, but a small priority was placed on studying the health effects of the materials involved in the project.