Leon Trotsky, co-leader of the October 1917 Revolution, was assassinated in Mexico on August 20, 1940. Security and the Fourth International, launched by the International Committee of the Fourth International launched in May 1975, was the first major investigation into the circumstances of his murder. This volume presents documents from the investigation’s first year, which uncovered abundant and long-concealed evidence of GPU penetration of the Trotskyist movement. These revelations exposed how Stalin’s agents orchestrated the murders of Trotsky, his son Leon Sedov, and comrades including Rudolf Klement, Erwin Wolf, and Ignace Reiss, as part of the broader counterrevolution that followed the Moscow Trials. The investigation details how embedded agents enabled the assassinations of Trotskyist cadres and facilitated Trotsky’s killing, arguing that exposing these state operations is essential to defending the political continuity of Marxism and the Fourth International.