At Harwell, Pontecorvo continued to be involved in reactor 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.