Stalin secret police what four letter acronym were they called in his time

The secret police became the primary pillar of Stalin's personal dictatorship, that is Stalin’s secret police. OGPU was the secret police which sometimes was a part of NKVD (which means Narodny Komissariat Vnutrennih Del, that is People's Comissariat of Internal Affairs), and sometimes was not. The OGPU was reorganized under the acronym NKVD in 1934. 

which time period is this? or is this story fictional, i'm going to need more info to answer this question