Callen was assigned by Dr. Gregory Rabinowitz to go to New York City and infiltrate the Trotskyist Socialist Workers Party (SWP) using the pseudonym "Sylvia Caldwell".
Venona messages reveal Callen giving copies to the KGB of SWP correspondence, intimate information on Leon Trotsky's widow, Natalia Sedova, and financial reports of Trotskyist groups.
Invoking the Fifth Amendment, she refused to answer questions about her membership in the SWP, her relationship with the KGB, Louis Budenz, or anything else.
This time, she was more cooperative, confessing that she met regularly with Rabinowitz and Soble to pass on confidential Trotskyist material at an apartment rented by a woman named Lucy Booker.
[1] It was intended to decrypt messages transmitted by the intelligence agencies of the Soviet Union (e.g. the NKVD, the KGB, and the GRU).