There wasn’t just one spy at Los Alamos. At least four people working inside the secret nuclear weapons laboratory during World War II passed classified information to the Soviet Union: Klaus Fuchs, Theodore Hall, David Greenglass, and Oscar Seborer. Of these, Fuchs was by far the most damaging, providing detailed bomb designs that helped the Soviets build their first nuclear weapon years ahead of schedule.
Klaus Fuchs: The Most Damaging Spy
Klaus Fuchs was a German-born physicist who had fled to Britain after the Nazis came to power. He was a committed communist, and he volunteered his services to Soviet intelligence before he ever set foot in the United States. For two years while working on uranium enrichment methods in New York, he met with a Soviet handler six times, passing along information about gaseous diffusion, one of the key processes for producing bomb-grade uranium.
In August 1944, Fuchs arrived at Los Alamos. He worked in the Theoretical Division under Hans Bethe, focusing on the design of the implosion bomb, the more complex of the two weapon types being developed. His access to sensitive information was extraordinary. By February 1945, he was passing along details about the critical mass needed for a plutonium weapon and the core design concepts behind the implosion approach. In June 1945, just weeks before the first nuclear test at Trinity, Fuchs handed over a sketch of the bomb itself, complete with component dimensions and a detailed written description.
After the war, Fuchs also shared information about the “Super,” the early hydrogen bomb concept. Soviet authorities found this useful for their own thermonuclear program, though historians debate exactly how much it accelerated their work. Fuchs was arrested in England in 1950 after decrypted Soviet communications pointed investigators in his direction. He was convicted and sentenced to 14 years in prison, the maximum allowed under British law at the time.
Theodore Hall: The Youngest Spy
For more than four decades, Fuchs was believed to be the only physicist who had spied at Los Alamos. That changed in the mid-1990s when the release of the Venona intercepts, a massive U.S. effort to decrypt Soviet wartime communications, revealed a second scientist-spy: Theodore Hall. Hall was just 18 when he arrived at Los Alamos and only 19 when he made contact with Soviet intelligence in November 1944.
Like Fuchs, Hall was a volunteer motivated by ideology rather than money. The information he provided was less detailed and less voluminous than what Fuchs handed over, but it covered implosion design and other aspects of the weapons program. For Soviet intelligence, Hall’s material served as an important supplement and confirmation of what Fuchs was providing. Having two independent sources allowed the Soviets to cross-check the accuracy of what they were receiving.
Hall was never prosecuted. The U.S. government detected his espionage through the Venona decrypts but lacked enough corroborating evidence to bring a case to trial without revealing the existence of the top-secret decryption program. Hall lived freely until his death in 1999, eventually confirming his espionage in interviews late in life.
David Greenglass and the Rosenberg Ring
David Greenglass was not a scientist. He was an Army sergeant and skilled machinist stationed at Los Alamos, assigned to a team making molds for the high-explosive lenses used to trigger the plutonium bomb’s implosion. In November 1944, Greenglass and his wife were recruited to spy by his brother-in-law, Julius Rosenberg, who was already running a Soviet espionage network.
By mid-1945, Greenglass had sent Rosenberg a crude sketch of the implosion lens along with twelve pages of detailed notes on the bomb design. While his understanding of the physics was limited compared to Fuchs or Hall, his hands-on work with the bomb’s physical components gave him practical knowledge that filled in gaps. Greenglass pleaded guilty in 1950 and received a 15-year sentence. He was released from prison in 1960. His cooperation with prosecutors came at a devastating personal cost: he testified that his sister, Ethel Rosenberg, had typed the notes Julius passed to the Soviets. Julius and Ethel Rosenberg were executed in 1953.
Oscar Seborer: The Fourth Spy
For seven decades, the FBI knew about a fourth spy at Los Alamos but kept the identity buried in investigative files. The name finally surfaced publicly after documents were declassified in 2011, scattered across a few dozen easily overlooked pages among tens of thousands of FBI records. The fourth spy was Oscar Seborer.
Seborer was an Army technician, not a scientist, but he had university-level engineering training. He worked at the Oak Ridge uranium facility before transferring to Los Alamos in 1944, where he remained until 1946. He was present at the Trinity test site as part of a unit monitoring seismological effects of the world’s first nuclear explosion. Like Greenglass, his position as a technician gave him access to sensitive material despite his relatively low rank.
The specifics of what Seborer passed to the Soviets remain partially unclear, but one detail speaks to its significance: Soviet authorities awarded him the Order of the Red Star in 1964, a military decoration that suggests his contribution was considered substantial. The first hint the FBI received of his involvement came in 1954, years after the war ended. Rather than face investigation, Seborer and his brother eventually defected to the Soviet Union, where they lived out their lives.
How They Got Away With It
Wartime security at the Manhattan Project sites was strict by the standards of the day but riddled with blind spots. Workers wore badges with letter designations (A through D) that controlled who they could speak with about the project. Colored armbands indicated which buildings and areas a person could enter. A formal Personnel Security Questionnaire screened applicants for loyalty and personal history. But these measures were designed to catch careless talk and unauthorized movement, not ideologically motivated scientists and technicians who had legitimate access to classified material.
The rush to build the bomb also worked against security. The Manhattan Project needed brilliant physicists, and it needed them fast. Background checks sometimes couldn’t keep pace with hiring, and political sympathies that might have raised red flags in peacetime were overlooked when the priority was winning the war. Fuchs had known communist associations in Germany, but British intelligence cleared him, and American security relied on that judgment. Hall was a Harvard-educated prodigy whose youth and talent made him seem an unlikely risk.
How They Were Caught
The key to unraveling the espionage was the Venona project, a secret American effort to decrypt intercepted Soviet intelligence communications. From 1948 to 1951, Venona decrypts helped identify multiple Soviet spies, including Fuchs, Greenglass, and Julius Rosenberg. Fuchs’s arrest in 1950 led investigators to his American courier, who in turn led them to Greenglass and the Rosenbergs. The dominoes fell in sequence once Fuchs confessed.
Hall was also identified through Venona but, as noted, couldn’t be prosecuted without exposing the program. Seborer’s espionage surfaced through an entirely different channel, a 1954 conversation captured by an FBI informant, and by then the trail had gone cold.
How Much Did the Spies Matter?
The Soviet Union tested its first atomic bomb in August 1949, years earlier than American officials had predicted. Historians estimate that espionage from Los Alamos allowed the Soviets to develop their bomb six months to two years faster than they would have managed on their own. Soviet scientists were talented enough to have eventually solved the problem independently, but the stolen information let them skip dead ends and confirm that their own calculations were on the right track. The implosion bomb design that Fuchs described in such detail was particularly valuable, since it was the more technically challenging of the two approaches and the one the Soviets chose to replicate for their first test.
The practical consequence was that the United States lost its nuclear monopoly sooner, reshaping the early Cold War and setting the stage for the arms race that defined the next four decades.