Simplify the following text for a Grade 9 native English-speaking person.
Original: In 1934 , the NKVD of the RSFSR was transformed into an all-union security force , the NKVD of the USSR ( which the Communist Party of the Soviet Union leaders soon became to call `` the leading detachment of our party '' ) , and the OGPU was incorporated into the NKVD as the Main Directorate for State Security ( GUGB ) ; the separate NKVD of the RSFSR was not resurrected until 1946 ( as the MVD of the RSFSR ) .
Simplified: The NKVD is best known for the Main Directorate for State Security ( GUGB ) , which came after the OGPU and the Cheka as the secret police agency of the Soviet Union , and which became KGB . Many think of the NKVD as a criminal group .