The secret of the first the atomic bomb developed by the United States during World War II was fiercely protected. Most of those caught passing on nuclear secrets were jailed and two were put to death. But one atomic spy evaded any consequences although he was known to authorities. He was left alone because the United States feared his prosecution would tip off the Soviets that they had cracked their coded communications, as that was the the only evidence against him. When the communications were declassified he was publicly identified as Theodore "Ted" Hall.
Hall worked on the Nagasaki weapon, a plutonium bomb, the same type that the Soviets detonated as their first nuclear test in 1949.