Doctors who carried out some of the government radiation experiments on unwitting citizens of the USA during the Cold War were also studying potential military application of radiological poisons. After feeding rats radioactive elements, in the 1940's one doctor secretly used three terminally ill human patients as experimental subjects to test lethal doses of plutonium. There is fragmentary evidence to suggest that other radioactive substances (including polonium, americium and radium) were injected into other, as yet unidentified subjects. One result of his work was to suggest to the military the use of radioactive smoke (fission product aerosols) as a killing agent for urban populations. (He sought healthy human volunteers to inhale near-lethal doses of radioactive aerosols, but found none. Having received an Atomic Energy Citation for "inspired, effective and pioneering leadership", he died in 1957, aged 49, of a rare form of leukaemia almost certainly caused by exposure to radiation).