The fourth atomic spy

Some say recently discovered Oscar Seborer was the most-damaging spy. Laboratory documents suggest otherwise.

By Alan Carr & Ellen McGehee | July 26, 2021

Seborer Horiz
Oscar Seborer Los Alamos National Laboratory

It’s been long known that Klaus Fuchs, Theodore Hall, and David Greenglass committed espionage at Project Y—the Los Alamos branch of the Manhattan Project—during World War II. Each worked at the secret laboratory charged with creating the world’s first atomic bombs, each stole classified weapons information, and each shared it with the Soviet Union. Just recently though, in September 2019, historians confirmed a fourth wartime spy: Oscar Seborer.

Seborer was born in New York City in 1921. He joined the Army in 1942 and, as a member of Army’s Specialized Training Program, he was sent to The Ohio State University to study electrical engineering. In 1944, given his academic training, Seborer was assigned to the Army’s Special Engineering Detachment, which provided specially trained soldiers to the Manhattan Project. Private Seborer was first sent to Oak Ridge, Tennessee, but by December 1944, he had been transferred to Los Alamos, New Mexico, where he began working for the Detonator Circuit (X-5) group in the Explosives (X) Division.

Seborer’s group was tasked with developing electrical equipment for measuring explosives tests and the firing circuits to ignite an implosion bomb’s detonators. (Detonators are small devices that ignite the high explosives surrounding the core of a nuclear weapon. The resulting explosion compresses—implodes—the core, which creates nuclear yield.) Significant progress was made on the detonator circuit in early 1945, so in April, freshly promoted Technician Fifth Grade Seborer was loaned to the Research Division to help prepare for the upcoming Trinity test, which would be the first successful detonation of a nuclear bomb.

Research on Seborer and his role at the Lab is ongoing, but we do know that part of his assignment included working on the rehearsal for Trinity, called the 100-Ton test. The same day the rehearsal was completed, May 7, Seborer’s X-5 group leader requested his immediate return. Although this was approved, 12 days later Trinity test director Kenneth Bainbridge asked X-5 to return Seborer to the Research Division by mid-June because he was “extremely valuable” to their work. Seborer was in high demand for his electrical knowledge and was sent back to the Trinity site in southern New Mexico to support earth shock experiments associated with the Trinity test, which was scheduled for July 1945.

Four Spies
Clockwise from top left: Los Alamos spies Klaus Fuchs, Theodore Hall, Oscar Seborer, and David Greenglass. In 2019, Seborer's story was unearthed by Harvey Klehr, a retired professor from Emory University, and John Earl Haynes, former historian for the Library of Congress.

As of March 1945, Seborer’s name was on a list of Los Alamos personnel proposed for the Destination Program, which was tasked with preparing atomic bombs for deployment to Japan. However, by June 1945, his name had been removed from the personnel list, probably due to his work supporting the Trinity test. Soon after the war, X-5 (along with Seborer) was transferred to Z Division, which had inherited many of the Destination Program’s responsibilities. By September 1945, Seborer had been promoted to Technician Fourth Grade and was working as an electrical technician in Z Division. In early 1946, like many in the wartime armed forces, Oscar Seborer was discharged from the military.

Even after his two promotions, Seborer only had a limited view of the overall project. He likely knew a considerable amount about the implosion bomb’s firing circuit, and he would have known something about diagnostic measuring equipment and techniques. Because he may have participated in the Destination Program and because he worked in Z Division, Seborer may also have known about the general concept of implosion and atomic bomb assembly procedures.

But, any knowledge Seborer had would have been eclipsed by that of his fellow mole, theoretical physicist Klaus Fuchs.

At Los Alamos, Fuchs was considered a technical staff member; he independently authored several reports and coauthored others with his division leader and future Nobel laureate Hans Bethe. Fuchs knew as much as anyone about the implosion bomb because he played a major role in its development.

Although Seborer’s treachery contributes to the story of Manhattan Project–era espionage, the prevailing narrative remains unchanged: The spies at Los Alamos collectively made a valuable contribution to the Soviet nuclear weapons program, and the information provided by Fuchs was almost certainly the most useful.

In 1950, Klaus Fuchs confessed and spent nearly a decade in prison. Shortly after, Julius and Ethel Rosenberg (sister of Los Alamos spy David Greenglass) were sentenced to death for committing espionage elsewhere on behalf of the Soviets. Through an informant, FBI investigators discovered Seborer was a spy in 1955. By then, however, he had already immigrated to the Soviet Union. He died in Moscow on April 23, 2015.