design projects. As a member of the Power Steering Committee (PSC), he was involved in discussions of the production and use of fissile materials, and of the materials used in construction of reactors. In 1949, other Via Panisperna boys, particularly Emilio Segrè, began to press their claim over patents relating to the behaviour of slow neutrons, which were at the heart of nuclear reactor—and nuclear weapon—design. The American Federal Bureau of Investigation (FBI) therefore began looking into their backgrounds. It noted, from its 1943 Manhattan Project files, that Pontecorvo's siblings Giuliana, Laura and Gillo were communists, and that Pontecorvo and Marianne were probably communists too, and reported this to MI5 in Britain, and MI6's Kim Philby in Washington, D.C. In February 1950, Pontecorvo's Harwell colleague Klaus Fuchs was arrested for espionage, and the AERE began to take security more seriously, and Pontecorvo was interviewed by Henry Arnold, the security officer at AERE. While Arnold had no evidence that Pontecorvo was a Soviet spy, he did feel that he was a security risk, and recommended that he be moved to a position where he did not have access to Top Secret material. Herbert Skinner suggested to Pontecorvo that he apply for a newly created professorship at the University of Liverpool, where Skinner held the Lyon Jones chair of experimental physics. In June 1950, Pontecorvo was offered the position. On 1 September 1950, in the middle of a holiday in Italy, Pontecorvo abruptly flew from Rome to Stockholm with his wife and three sons without informing friends or relatives. On 2 September he was helped by Soviet agents to enter the Soviet Union from Finland. His abrupt disappearance caused much concern to many of the western intelligence services, especially those of Britain, Canada and the United States, that were worried about the escape of atomic secrets to the Soviet Union, and of Finland and Sweden, through which Pontecorvo and Marianne had been allowed to travel without valid passports and visas. In 1952, Pontecorvo's potential role in the transfer of nuclear secrets to Russia was discussed in American newspapers. According to Oleg Gordievsky, the highest-ranking KGB officer ever to defect, and Pavel Sudoplatov, the former deputy director of Foreign Intelligence for the Soviet Union, Pontecorvo was a Soviet spy. However, Sudoplatov misidentified Pontecorvo as the spy codenamed Mlad, whom we now know was Ted Hall. While Pontecorvo always denied working on nuclear weapons,