In response to the 1939 Einstein-Szilard letter, President Franklin D. Roosevelt had created an Advisory Committee on Uranium in October 1939, chaired by Lyman Briggs. Research concentrated on slow fission for power production, but with a growing interest in isotope separation. In June 1941, Roosevelt created the Office of Scientific Research and Development (OSRD), with Vannevar Bush as its director, personally responsible to the President. The Uranium Committee became the Uranium Section of the OSRD, which was soon renamed the S-1 Section for security reasons.