Guépéou
Prior to April 1, 1922, investigation of the PSR leadership was conducted by the Cheka and its institutional successor, the State Political Directorate (, GPU). This investigation was headed by Iakov Agranov, a top leader of the secret police establishment, who hand-selected potential witnesses from the scores of PSR members that had been arrested and detained over the course of 1921 and the first months of 1922.