Ted Hall was a Soviet spy in Los Alamos National Laboratory in the 1940s. Unlike his peers, Klaus Fuchs, Alger Hiss, and Julius and Ethel Rosenberg, he has never been formally accused of the crime.
There is no doubt of his guilt; intercepted Soviet communications from the 40s talk about an agent at Los Alamos called “Mlad” (the young one), who on the basis of timing, location, and the information he was able to pass on, was Hall. As increasing amounts of evidence was released in 1996 and 1997, it became impossible to defend Hall. Some of the cables referred to "Teodor Kholl" and "Savil Saks" by name (Saville Sax had been Hall's best friend and college roommate, and later served as courier between him and the Soviets.)
The Venona Intercepts describe how Hall met with Sergey Kurnakov (codenamed 'Bek' in the cable), a Soviet journalist and KGB agent living in New York. Note that this meeting was the idea of Hall and Sax; they approached the Soviets, not the other way around. Kurnakov reported that Hall had "an exceptionally keen mind and a broad outlook, and is politically developed," meaning he was a committed Communist. At his first meeting with Kurnakov, Hall handed over documents that provided specifics both of research and job responsibilities at the Manhattan Project.
In its initial release of the Venona documents in 1995, the National Security Agency blotted out the identity of 'Mlad'. But the cables supplied enough information for Washington Post reporter Michael Dobbs to break the story of Hall's involvement.
In a written statement published in 1997, he came close to admitting the charges, although obliquely, saying that in the immediate postwar years, he felt strongly that "an American monopoly" on nuclear weapons "was dangerous and should be avoided."
To help prevent that monopoly I contemplated a brief encounter with a Soviet agent, just to inform them of the existence of the A-bomb project. I anticipated a very limited contact. With any luck it might easily have turned out that way, but it was not to be.
He apparently gave them a lot more than just knowledge of the existence of the Manhattan Project (which they had anyway). He gave them substantial technical documentation on the implosive lens, accelerating their own atomic bomb project by years.
The FBI knew this since about 1950 but never arrested him.
He could have been famous for other things. He was brilliant when young. By the time he was 14, he was tentatively admitted to Columbia University. He switched to Harvard University at 16, and had he degree at the age of 18. He was the youngest of the team of young scientists recruited to the Manhattan Project.
Hall went on to get a doctorate in physics from the University of Chicago, then took a job at the Sloan-Kettering Institute for Cancer Research in New York. In 1962, he and his wife left for what was initially to be one year at the Cavendish Laboratory at Cambridge University. There he worked on electron microscopes in biological X-ray micro-analysis.
Hall died on November 1, 1999 in Cambridge, England. He was 74 years old.
Washington Post Weekly, March 11, 1996
New York Times, November 10, 1999