Cairncross worked as private secretary to Sir Maurice Hankey, a high-ranking British official involved with Tube Alloys, the secret British atomic program 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, Cairncross gave information in exchange for immunity from prosecution. In 1990, he was finally identified as the “fifth man” in the infamous group of spies (including Kim Philby, Guy Burgess, Donald Maclean and Anthony Blunt) who met at Cambridge University in the 1930s. Cairncross died on October 8, 1995, in Herefordshire, England.

The Soviet Union’s longest-serving spy in Britain, Norwood worked as a secretary for a director of the Tube Alloys project. While living an apparently normal life in the London suburbs, she passed information to Soviet agents throughout the war—and into the 1970s. It’s unclear how much Norwood’s espionage helped the Soviet atomic program, but she was officially honored for her work when she visited Moscow in 1979. Finally exposed as a spy in the 1990s, Norwood “cheerfully admitted what she had done, and said she would do it again,” says Harvey Klehr, emeritus professor of politics and history at Emory University and author of various books on Soviet espionage.

Fuchs, a German-born physicist, fled to England amid the rise of Nazism in 1933 and became a British citizen in 1942. By that time, he had already offered to spy for the Soviets. In late 1943, Fuchs joined a group of British scientists who traveled to Los Alamos to work for the Manhattan Project, and he later passed key information about atomic weapons design to the Soviets that enabled them to accelerate their nuclear program. After decrypted cables revealed Fuchs’ espionage, he confessed in early 1950. His testimony led authorities to Harry Gold, an important courier for other Los Alamos spies.

Gold in turn named David Greenglass, a U.S. Army machinist who had worked at the classified nuclear facility at Oak Ridge, Tennessee before being assigned to Los Alamos in 1944. Recruited to spy for the Soviets by his brother-in-law, Julius Rosenberg, Greenglass passed information to the Soviets in mid-1945 that included a hand-drawn sketch and notes describing the implosion-type bomb. In his 1950 confession, Greenglass implicated his own sister, Ethel Rosenberg, whom he said had typed the notes that were sent to the Soviets. His cooperation earned him a lesser sentence and immunity for his own wife, Ruth. Based largely on the Greenglasses’ testimony, the Rosenbergs were convicted and executed in June 1953.

McNutt was a civil engineer in New York City and a friend of Julius Rosenberg, who in late 1943 encouraged him to get a job at Kellex, a company building the massive gaseous diffusion plant to separate uranium at Oak Ridge. Rosenberg connected McNutt to the KGB, the Soviet security agency. Though he gave the Soviets the plant's design, McNutt (despite Soviet pleas) refused Kellex’s offer to relocate from New York to Oak Ridge, where he would have had access to more scientific data.

“The FBI questioned him because he was friendly with Rosenberg, but they never suspected him of being a spy,” Klehr says. After the war, McNutt worked for Gulf Oil and led the company’s Gulf-Reston division, which built the planned community in Reston, Virginia—right next to the CIA’s Langley headquarters. McNutt’s espionage was later revealed in the notebooks of Alexander Vassiliev, a journalist and former KGB officer who was able to take notes on sensitive KGB archives dating to 1930-50.

Hiskey, a chemist, began working on gaseous diffusion at Columbia University and was later transferred to Chicago’s Metallurgical Laboratory (Met Lab), another key part of the Manhattan Project. Hiskey passed information to the GRU, or Soviet military intelligence, rather than the KGB. After he was seen meeting with the known Soviet agent Arthur Adams in 1944, U.S. Army intelligence officials drafted Hiskey into active duty (he had a reserve commission) and shipped him off to Alaska.

“They didn't want him arrested because if they had to charge him, it would expose the fact that he was working on this top-secret project,” Klehr says. Hiskey was called to testify before a congressional committee after the war but refused to answer questions about his suspected espionage. “They really didn't have any hard evidence,” Klehr points out. “So he got away with it.” Hiskey went on to teach chemistry at Brooklyn Polytechnic Institute and work at several biotechnology companies.

The release of the decrypted Venona intercepts in the mid-1990s revealed that Theodore Hall, the youngest physicist on the Manhattan Project, was the long-suspected third spy (after Fuchs and Greenglass) at Los Alamos. Codenamed “Mlad,” Hall had reached out to the Soviets in late 1944 and soon after provided them with a key update on the development of the plutonium bomb. The FBI had first learned of Hall’s spying activities in the early 1950s, but without a confession, the FBI had to let him go rather than reveal the Venona project to the Soviets. Hall later moved to Britain, where he became a pioneer of biological research.

In 2019, after searching through recently declassified FBI files, Klehr and John Earl Haynes reported the existence of a fourth Soviet spy at Los Alamos. Oscar Seborer, codenamed “Godsend,” was the son of Jewish immigrants from Poland who became an electrical engineer and worked at Los Alamos from 1944-46. Though it’s still unknown exactly what information Seborer provided to the Soviets, his work on the wiring of the bomb’s explosive trigger would have given him access to different information than Fuchs and Hall, including key intelligence about the implosion detonation method.

“At this point, we don’t know exactly what he provided,” says Klehr. “It could have been very significant—that’s about all we can say.” By the time the FBI learned of Seborer’s espionage in the mid-1950s, he had left the United States and settled in Russia, where he lived until his death in 2015.