The precise nature of Robert Oppenheimer's relationship with the NKVD has long been a topic of intense interest. Although Robert was never formally a member of the Party, practically all his friends were, and, as he subsequently admitted, he 'probably joined every Communist front on the West Coast'. Frank, who joined the Radiation Laboratory in 1941 from Stanford University (and worked at both Oak Ridge and Los Alamos), had been a member of the CPUSA's Palo Alto branch since 1937, as was Robert's wife, Kitty. Her first husband, Joseph Dallet, was a CPUSA official who had fought in Spain as a member of the international Brigade, and the news of his death in 1937 had been broken to her in Paris by one of his comrades, Steve Nelson. Her second husband, whom she married in December 1938, was an English physician, Richard Harrison, who was interested in radiation and travelled with her to Pasadena where she took a graduate course in mycology and met Robert Oppenheimer. After eighteen months of marriage, Kitty divorced Harrison, and married Oppenheimer, and this was how the latter came into contact with Nelson, a senior C P U S A functionary and an NK V D courier for Oppenheimer's students Joseph W. Weinberg, Giovanni Rossi Lomanitz and David Bohm who worked at the Radiation Laboratory at Berkeley.

Originally from New York, Weinberg had graduated from the City College and become an active member of the Young Communist League before moving to California. His wife Muriel, who had studied at the University of Wisconsin, was also a Party activist, and C P U S A literature mailed to Radiation Laboratory scientists was traced to her. In March 1943 FBI surveillance on Steve Nelson's home and telephone identified Weinberg as a late-night visitor who wanted to copy a formula written in the handwriting of another scientist. This had been followed soon afterwards by a covert rendezvous in a park between Nelson and a Soviet vice-consul, Piotr Ivanov, at which packages were seen to be exchanged, and then by Zubilin's visit to Nelson's home. In August Weinberg was watched by Counter-Intelligence Corps (CIC ) agents George Rathman and Harold Zindle while he hosted a clandestine Party gathering at his apartment in Black Street, Berkeley, attended by Nelson and Lomanitz. Weinberg always denied his membership of the Party, his involvement in espionage and any knowledge of Steve Nelson, despite surveillance evidence showing Nelson's secretary, Bernadette Doyle, was a visitor to his home. He was indicted on four counts of contempt after he had pleaded the Fifth Amendment before the House Un-American Activities Committee (HUAC). The charges were dismissed in March 1953 on the grounds that Weinberg had exercised his constitutional right, but nevertheless he lost his post at the University of Minnesota's Department of Physics.