This article focuses on M15 organization and Klaus Fuchs, a German-born physicist and Soviet “Atom Spy” who was arrested in 1950 and served fourteen years for offences related to atomic espionage. It examines how Fuchs was identified as an “Atom Spy” in 1949 and describes the MI5's investigation, which ended in the early 1950 with the successful arrest, prosecution, and imprisonment of this highly significant Cold War figure. Key issues discussed in this article include the difficulties encountered by MI5 and the budding British atomic program in the sphere of security. It also discusses the role of Signals intelligence (SIGINT) in the investigation of Fuchs, and the high-risk but ultimately successful approach taken by MI5's key interrogator, William Skardon. This case study highlights both the unparalleled level of international intelligence cooperation between the British agencies and their American counterparts, which made the resolution of this case possible, and some of the frailties in the Anglo-American alliance that were brought to the fore by the exposure of Fuchs as an Atom Spy.
