The Gulag was run at first by the GPU, later by the NKVD and in the final years by the Ministry of Internal Affairs (MVD). The internment system grew rapidly, reaching a population of 100,000 in the 1920s. According to Nicolas Werth, the yearly mortality rate in the Soviet concentration camps varied, reaching 5% (1933) and 20% (1942–1943) and dropped in the post-war years to about 1 to 3% per year at the beginning of the 1950s.