The name of a missing Soviet spy which was hidden from the public for the past 70 years has just been revealed.

The American man stole U.S. atomic bomb secrets along with three other known individuals.

This stealing of highly classified information between 1940 and 1948 was said to fast-track the Soviet's development of nuclear weapons and the start of the Cold War.

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For a long time, only three spies were blamed for this information leak but a man called Oscar Seborer has just been named as a fourth culprit.

His code-name was "Godsend" and he worked at the Los Alamos National Laboratory in New Mexico.

This is where the first nuclear weapons were designed.

Seborer's name was mentioned a few times in FBI documents that were declassified in 2011.

His betrayal of the U.S. came to the attention of historians John Earl Haynes and Harvey Klehr and the New York Times revealed that his story is finally being told.

The theory of a fourth spy being responsible for leaking atomic secrets along with David Greenglass, Klaus Fuchs and Theodore Hall, was proposed in the early 1990s.

However, this theory was thought to be based on a Russian misinformation campaign.

Haynes and Klehr looked into declassified FBI documents to confirm the theory.

They found out that Seborer was mentioned less in the documents because the focus was largely on the other spies.

Many questions remain about his activities as a spy and what happened when he defected to the USSR.

The historians did learn that he was from a Jewish family who had moved to the U.S. from Poland.

Seborer was part of a network of people connected to Soviet intelligence and some were known members of the Communist Party.

He trained as an engineer and enrolled in the U.S. Army in 1942.

He was then assigned to the Manhattan Project, a World War II project that created the first nuclear weapons.

During this time he is suspected to have been leaking secrets.

Seborer was reassigned to the US Navy two years later and worked as an electrical engineer.

The historians discovered that his senior officers had reported him as a "security risk" at the time but this was largely due to his associations with the Communist Party than spying suspicions.

Anti-Communism reached new heights in 1950s America.

Seborer decided to flee the country in 1952 with his brother, sister-in-law and mother-in-law.

He eventually settled in Moscow and died in 2015.

Seborer has even been accused of handing over the formula for the 'A' bomb due to documents from a Communist Party member that appear to mention him.

The specific atomic secrets that he shared are still being pieced together and the significance of Seborer's contributions will undergo further investigation by researchers.

It is thought that he didn't act alone and some of his family members were also spies with the code-names "Godfather," "Relative" and "Nata."

This story originally appeared in The Sun.