Groves and the wartime director of the Los Alamos Laboratory, Robert Oppenheimer, had begun the move of ordnance functions to Sandia in late 1945.[31] The laboratory's ordnance engineering division, known as Z Division, after its first director, Jerrold R. Zacharias,[32] was split between Los Alamos and Sandia. Between March and July 1946, Z Division relocated to Sandia, except for its mechanical engineering (Z-4) section, which followed in February 1947.[33] Z Division worked on improving the mechanical and electrical reliability of the Mark 3 Fat Man bomb, but this work was disrupted by the Crossroads tests.[34]