There wasn’t one spy in the Manhattan Project. There were at least seven, all working for the Soviet Union. The most damaging was Klaus Fuchs, a British physicist who worked in the Theoretical Division at Los Alamos and passed detailed atomic weapons designs to Soviet handlers starting in 1941. Together, these spies sent enough information back to Moscow to significantly accelerate the Soviet Union’s development of its own atomic bomb, which it successfully tested in August 1949.
Klaus Fuchs: The Most Valuable Spy
Klaus Fuchs was a German-born physicist who fled Nazi Germany and settled in Britain, where he became a committed communist. He first offered his services to Soviet intelligence in late 1941, initially passing along information about British atomic research. When the British sent a team of scientists to join the Manhattan Project, Fuchs came with them and was assigned to the Theoretical Division at Los Alamos, the heart of the bomb design effort.
From that position, Fuchs passed detailed information about atomic weapons design to his Soviet handlers. He understood the science at its deepest level, making his intelligence far more valuable than what any lower-ranking source could provide. His courier was Harry Gold, an American who had been spying for the Soviets since 1936 and served as the link between Fuchs and Soviet intelligence. Fuchs was not caught during the war. It took years of codebreaking before investigators identified him, and he was finally arrested in 1950 in Britain, where he confessed and served nine years in prison.
Theodore Hall: The Youngest Spy
Theodore Hall was just 18 years old when he arrived at Los Alamos, one of the youngest physicists on the project. A long-time communist sympathizer, he voluntarily made contact with Soviet intelligence in November 1944. His motivation appears to have been ideological: he believed no single country should hold a monopoly on such a devastating weapon.
The information Hall provided was not as detailed or voluminous as what Fuchs gave, but it covered implosion technology and other aspects of weapons design. For the Soviets, Hall’s intelligence served as an important supplement and confirmation of Fuchs’s material, essentially giving them two independent sources describing the same bomb. Hall’s identity as a spy remained hidden until the mid-1990s, when the U.S. government declassified the VENONA intercepts, a collection of decoded Soviet intelligence messages. By that point, too much time had passed to prosecute him, and he was never charged.
David Greenglass and the Rosenberg Connection
David Greenglass was an Army machinist, not a physicist, but his hands-on role at Los Alamos gave him access to critical hardware. He was briefly assigned to Oak Ridge, Tennessee, in the summer of 1944 before transferring to Los Alamos, where he worked on the implosion bomb as part of the Special Engineering Detachment. Specifically, his team made molds for the high-explosive lenses used to detonate the plutonium core.
In November 1944, Greenglass and his wife Ruth were recruited to spy by his brother-in-law, Julius Rosenberg. Using Ruth as a conduit, Greenglass funneled information to Rosenberg, who passed it to Soviet intelligence. In mid-1945, Greenglass sent along a crude sketch of the implosion bomb and twelve pages of detailed notes. Decoded VENONA cables later revealed that Greenglass and his wife had been given the codenames KALIBR (Caliber) and OSA (Wasp).
The Greenglass case became the most publicly dramatic chapter of Manhattan Project espionage. In February 1951, Greenglass testified against his own sister, Ethel Rosenberg, stating that she had typed the information Julius later handed off to the Soviets. His testimony proved decisive. Julius and Ethel Rosenberg were convicted and executed in the electric chair on June 19, 1953. Greenglass received a 15-year sentence in exchange for his cooperation.
Oscar Seborer: The Fourth Los Alamos Spy
For decades, historians knew of three wartime spies at Los Alamos: Fuchs, Hall, and Greenglass. Then, in September 2019, researchers confirmed a fourth: Oscar Seborer. A U.S. Army private, Seborer was first sent to Oak Ridge but by December 1944 had been transferred to Los Alamos. He worked in the Detonator Circuit group within the Explosives Division, giving him direct knowledge of how the bomb’s triggering mechanism functioned. Seborer eventually fled the United States and defected to the Soviet Union, where his espionage remained largely unknown to the American public for over 70 years.
George Koval: Inside the Production Sites
Most Manhattan Project spies operated at Los Alamos, where the bombs were designed. George Koval was different. He infiltrated the production facilities where the raw materials for the bomb were actually made, and he was the only Soviet spy with top-secret clearance granting unrestricted access to those sites.
Unlike the ideologically motivated scientists, Koval was a trained intelligence officer, recruited and prepared by the Soviet military intelligence directorate (the G.R.U.) before being inserted into the U.S. Army. From 1944 to 1946, he worked at Manhattan Project sites in Oak Ridge, Tennessee, and Dayton, Ohio. At Oak Ridge, he served as a Health Physics Officer, a role that required him to monitor radiation levels across the entire sprawling complex. “He had access to everything,” recalled Arnold Kramish, a fellow worker at a different Oak Ridge lab.
Koval reported back to Moscow on the existence of Oak Ridge’s massive enrichment and production facilities, including the K-25, Y-12, and X-10 sites. He provided details about how uranium and plutonium were produced and how materials were transported between Manhattan Project locations. He also tracked Oak Ridge’s polonium production, a critical component of the bomb’s initiator. His intelligence had a direct strategic impact: Soviet nuclear chief Igor Kurchatov, learning from Koval’s reports that uranium enrichment facilities were too massive for the Soviet industrial base to replicate quickly, made the decision in 1945 to focus on building a plutonium bomb instead. Koval’s contributions remained secret until 2007, when Russian President Vladimir Putin posthumously awarded him the Hero of the Russian Federation medal, calling him the “only Soviet intelligence officer to penetrate the U.S. secret atomic facilities producing the plutonium, enriched uranium and polonium used to create the atomic bomb.”
Spies at Chalk River
Two additional scientists spied for the Soviets from the Chalk River nuclear facility in Ontario, Canada, which was part of the broader Allied atomic effort. Allan Nunn May was a British physicist who arrived in 1943 as part of James Chadwick’s team. Unlike Fuchs, he was not assigned to Los Alamos but instead helped build a heavy water reactor at Chalk River. In February 1945, he passed what he had learned to Soviet intelligence. His colleague Bruno Pontecorvo, an Italian physicist and former protégé of Enrico Fermi, also established contact with Soviet handlers and fed them information about the Chalk River operation. Pontecorvo later defected to the Soviet Union in 1950.
How They Were Caught
The key to unraveling Manhattan Project espionage was VENONA, a secret U.S. Army signals intelligence program that worked to decrypt Soviet diplomatic communications. The coded messages, painstakingly decoded over years, contained references to spies operating inside the atomic program. VENONA led investigators to Fuchs, whose 1950 confession then led to Harry Gold, who in turn identified David Greenglass, which ultimately unraveled the Rosenberg network.
The first batch of translated VENONA messages was publicly released in July 1995 and included 49 messages specifically about Soviet efforts to penetrate the Manhattan Project. Over subsequent releases, approximately 3,000 total VENONA translations were made public, revealing the full scope of Soviet wartime espionage. Theodore Hall’s role only became publicly known through these releases, decades after the war ended. George Koval and Oscar Seborer remained hidden even longer, their identities not confirmed until 2007 and 2019 respectively.
Why They Did It
What’s striking about the Manhattan Project spies is that most were not paid agents or coerced informants. They were volunteers driven by ideology. Fuchs and Hall were both committed communists who offered their services to Soviet intelligence on their own initiative. They believed, broadly, that sharing atomic secrets would prevent a dangerous American monopoly on nuclear weapons and promote global balance. Greenglass was recruited through family ties and ideological sympathy, pulled into espionage by Julius Rosenberg. Harry Gold, the courier, later said his upbringing had instilled a deep belief in helping others, which Soviet recruiters exploited to draw him into espionage as far back as 1936. Koval was the exception: a professional intelligence officer sent by the Soviet military with a specific mission.
Collectively, these spies gave the Soviet Union a remarkably complete picture of the American atomic program, from theoretical bomb design at Los Alamos to production methods at Oak Ridge to reactor technology at Chalk River. Historians generally estimate the intelligence shaved years off the Soviet timeline, helping the U.S.S.R. test its first bomb in 1949, just four years after Hiroshima.