Theodore Alvin Hall (1925-1999) was a teenager hired to work on the Manhattan Project, who passed top-secret details about the "Fat man" plutonium bomb to the Soviet Union.
Hall was a physics prodigy who graduated Harvard at age 18, where he had become a good friend of another communist sympathizer. On a vacation back in New York, he visited the Soviet consulate and initiated contact to pass atomic secrets.
Hall was questioned by the FBI in 1951 but wasn't charged due to FBI decision that the Venona project would be inadmissible hearsay and not worth compromising the program.
Hall switched to biology after the Manhattan Project and eventually settled in England.