The Story of Oscar Seborer, Atomic Spy

It’s been long known that Theodore “Ted” Hall, Klaus Fuchs, and David Greenglass committed espionage at Los Alamos during World War II.

Each worked at the secret Laboratory where the world’s first atomic bomb was under development, and stole classified weapons information, sharing it with the Soviet Union.

Just recently though, in September 2019, historians confirmed a fourth wartime spy: Oscar Seborer. But who was he, what do we know about him, what did he steal, and what happened to him?

Working at the Lab

Seborer was born in New York City in 1921. Little is currently known of his early life, but when WWII started, he was studying electrical engineering at Ohio State University. He joined the Army in the fall of 1942 and, given his academic training, was assigned to the Army’s Special Engineering Detachment (SED). Private Seborer was initially sent to Oak Ridge; in December 1944, he transferred to Los Alamos.

Late in 1944, the Explosives Division (X) was struggling with detonator development. Seborer’s Group, Detonator Circuit (X-5), was tasked with developing electrical equipment for measuring explosives tests and the firing circuits to ignite the implosion bomb’s detonators. Significant progress was made on the detonator circuit in early 1945, so in April, freshly promoted Technician Fifth Class (Corporal) 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.

Part of Seborer’s assignment included working on the rehearsal for Trinity, the 100-Ton Test. The same day the rehearsal was completed, May 7, Seborer’s group leader in X-5 requested his immediate return. Although this was approved, only a dozen days later Trinity’s Test Director Kenneth Bainbridge asked X-5 to return Seborer to the Research Division no later than mid-June because he was “extremely valuable” to their work. Although he had only spent a year and a half as a private, that summer Seborer was promoted for the second time in six months, this time to Technician Fourth Class (Sergeant). It’s clear Seborer was in high demand and also highly thought of by his managers.

Seborer asked to be transferred to the Destination Program in May 1945. The program was tasked with preparing the atomic bombs for deployment to Japan, but it’s unclear if Seborer’s request was granted. After the war, however, X-5 was transferred to Z Division, which had inherited many of the Destination Program’s responsibilities. Only a few years later, Z Division became an independent organization – what is today Sandia National Laboratories. 

Hardly the Father of the Soviet Atomic Bomb

A January 27, 2020, New York Times article proclaimed Seborer’s “knowledge most likely surpassed that of the three previously known Soviet spies at Los Alamos, and played a crucial role in Moscow’s ability to quickly replicate the complex device.”

However, records in the National Security Research Center (NSRC), LANL’s classified library, do not support that conclusion.

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 something about the general concept of implosion and assembly procedures for the atomic bombs.

But, any knowledge Seborer would have been privy to would have been greatly eclipsed by that of his fellow mole, Klaus Fuchs.

Fuchs was a senior theoretical physicist. Like Seborer, Fuchs worked at Oak Ridge before joining the Los Alamos staff. Though nothing is known about Private Seborer’s work at Oak Ridge, it is known that Fuchs helped design the control system for the gaseous diffusion plant for enriching uranium.

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.

Evading prosecution, but not contempt

Though the discovery of Seborer’s treachery is an immense contribution to the story of Manhattan Project–era espionage, the prevailing narrative remains unchanged: Fuchs was, by far, the most damaging spy of wartime Los Alamos.

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, and Seborer quietly slipped out of the country.

Through an informant, FBI investigators discovered Seborer was a spy in 1955. By then, however, he had already immigrated to the Soviet Union.

Thus, the story of Oscar Seborer remained buried in classified FBI files until it was unearthed by professor Harvey Klehr and historian John Earl Haynes just months ago.

Records from the NSRC provide a technical context for Seborer’s tale, which came to an end April 23, 2015, with his death in Moscow.