Despite being allies during World War II, the Soviet Union launched an all-out espionage effort to uncover the military and defense secrets of the United States and Britain in the 1940s.

Within days of Britain’s highly classified decision in 1941 to begin research on building an atomic bomb, an informant in the British Civil Service notified the Soviets.

As the top-secret plan to build the bomb, called The Manhattan Project, took shape in the United States, the Soviet spy ring got wind of it before the FBI knew of the secret program’s existence.

Barely four years after the United States dropped two atomic bombs on Japan in August 1945, the Soviet Union detonated its own in August 1949, much sooner than expected. The Soviets had no trouble finding people to spy for them.

So, what made these Americans and Brits, who had gone to college, sell their countries’ atomic secrets?

Some did it because they believed in communist ideas and were passionate about them. The idea of nuclear balance drove others. They thought that one way to stop a nuclear war was to make sure that no country had a monopoly on this awesome power. For a long time, no one knew how far Soviet spies went.

In 1946, the United States and Britain worked together to figure out the code that Moscow used to send telegraph lines. This was the first step towards the big breakthrough; the decoding project was called Venona.

It was kept hidden until 1995, when it was made public because government officials didn’t want to let people know that they had broken the Russian code. For non-approved use, it couldn’t be used in court.

However, it could lead to investigations and surveillance to catch spies in the act or get them to admit to spying. As the ability to break the Venona code got better in the late 1940s and early 1950s, it gave away the identities of several spies.

No one knows how many spies got away. After investigations, at least a dozen people who had given atomic secrets to the Soviets were put to death or in jail. Here are the seven main spies who told the Soviets about the atomic bomb:

1. Klaus Fuchs

Klaus Fuchs was a scientist born in Germany. When the Nazis came to power in 1933, he fled to England. By 1942, he became a British citizen, and by that time, he had already agreed to spy for the Soviets. In late 1943, Fuchs went to Los Alamos with a group of British scientists to work on the Manhattan Project.

How Did the Soviets Acquire the Knowledge to Build an Atomic Bomb?

He eventually gave the Soviets important information about how atomic weapons were made, which helped them speed up their nuclear program. After decrypted messages showed that Fuchs was a spy, he admitted it at the beginning of 1950. His evidence helped the government find Harry Gold, who was a key messenger for the other Los Alamos spies.

2. John Cairncross

How Did the Soviets Acquire the Knowledge to Build an Atomic Bomb?

John Cairncross worked as a private secretary to a British official involved with the secret Atomic program known as Tube Alloys Project during World War II. In this position, he gave Moscow a list of American atomic scientists and may have leaked information about a report evaluating Britain’s prospects of building a uranium bomb in 1941. After he was interrogated by MI5 in the 1960s and confessed to being a Soviet spy, Cross gave information in exchange for immunity from prosecution. In 1990, he was finally identified as one of the infamous group of spies who met at Cambridge University in the 1930s. Cross died on October 8, 1995.

3. Melita Norwood

How Did the Soviets Acquire the Knowledge to Build an Atomic Bomb?

The Soviet Union’s longest-serving spy in Britain, Norwood, worked as a secretary for the director of the Tube Alloys Project. She seemed to live a normal life in the suburbs of London, but throughout the war and until the 1970s, she gave information to Soviet spies. It is not clear how much Norwood’s spying helped the Soviet Atomic program, but when she went to Moscow in 1979, she was given a formal award for her work. When she was found out as a spy in the 1990s, Norwood gladly admitted what she had done and said she would do it again.

4. David Greenglass

How Did the Soviets Acquire the Knowledge to Build an Atomic Bomb?

David Greenglass was a machinist in the U.S. Army before being sent to Los Alamos in 1944. He worked at a secret nuclear plant in Oak Ridge, Tennessee. Greenglass’s brother-in-law convinced him to spy for the Soviets. In the middle of 1945, he gave the Soviets a hand-down sketch and notes about the implosion-type bomb. Greenglass said that his sister wrote the notes that were sent to the Soviets in his confession from 1950. He got a shorter term, and his wife was not charged because he helped. The family members were found guilty and killed in June 1953.

5. Clarence Hiskey

How Did the Soviets Acquire the Knowledge to Build an Atomic Bomb?

Hiskey, a scientist, started working on gaseous diffusion at Columbia University. Later, he moved to the Metallurgical Laboratory in Chicago, which was also a key part of the Manhattan Project. He told the Soviet Military Intelligence, not the KGB, what he knew. After he was seen meeting with the known Soviet agent in 1944, U.S. Army intelligence officials put Hiskey on active service and sent him to Alaska. They didn’t want to arrest him because charging him would show that he was working on this top-secret project. After the war, Hiskey was asked to appear in front of a congressional committee, but he refused to answer questions about whether or not he had been a spy. They didn’t really have any hard evidence, so he didn’t get caught.

6. Theodore Hall

How Did the Soviets Acquire the Knowledge to Build an Atomic Bomb?

In the mid-1990s, the decrypted Venona intercept showed that Theodore Hall, who was the youngest scientist working on the Manhattan Project, was a main spy at Los Alamos. Hall called out to the Soviets in late 1944, under the code name “mlad,” and soon after, he gave them important information about the progress of the plutonium bomb. When the FBI found out about Hall’s spying in the early 1950s, they had to let him go because they didn’t want to tell the Soviets about the Venona project. Hall later went to Britain, where he was one of the first people to study biology.

7. Oscar Seborer

How Did the Soviets Acquire the Knowledge to Build an Atomic Bomb?

Oscar Seborer was an electrical engineer who worked at Los Alamos from 1944 to 1946. He was the son of Jewish immigrants from Poland. Even though we don’t know exactly what information he gave to the Soviets, the fact that he worked on connecting the bomb’s explosive trigger would have given him access to a lot of information, including key information about how the implosion was set off. By the time the FBI found out about Seborer’s spying in the middle of the 1950s, he had already left the U.S. and moved to Russia, where he stayed until his death in 2015.”