The Katyn massacre (Polish: zbrodnia katyńska, "Katyń crime"; Russian: Катынская резня Katynskaya reznya, "Katyn massacre", or Russian: Катынский расстрел, "Katyn execution by shooting") was a series of mass executions of about 22,000, Polish military officers and intelligentsia carried out by the Soviet Union, specifically the NKVD ("People's Commissariat for Internal Affairs", the Soviet secret police) and their executioner Vasily Blokhin, in April and May 1940. Though the killings also occurred in the Kalinin and Kharkiv prisons and elsewhere, the massacre is named after the Katyn Forest, where some of the mass graves were first discovered.
The massacre was initiated in NKVD chief Lavrentiy Beria's proposal of 5 March 1940 to execute all captive members of the Polish officer corps, approved by the Soviet Politburo led by Joseph Stalin. Of the total killed, about 8,000 were officers imprisoned during the 1939 Soviet invasion of Poland, another 6,000 were police officers, and the remaining 8,000 were Polish intelligentsia the Soviets deemed to be "intelligence agents, gendarmes, landowners, saboteurs, factory owners, lawyers, officials, and priests". The Polish Army officer class was representative of the multi-ethnic Polish state, the murdered also included Polish Ukrainians, Belarusians, and Polish Jews including the Chief Rabbi of the Polish Army, Baruch Steinberg.
The government of Nazi Party announced the discovery of mass graves in the Katyn Forest in April 1943. When the London-based Polish government-in-exile asked for an investigation by the International Committee of the Red Cross, Stalin immediately severed diplomatic relations with it. The USSR claimed the Nazis had killed the victims, and it continued to deny responsibility for the massacres until 1990, when it officially acknowledged and condemned the killings by the NKVD, as well as the subsequent cover-up by the Soviet government.
An investigation conducted by the office of the Prosecutors General of the Soviet Union (1990–1991) and the Russian Federation (1991–2004) confirmed Soviet responsibility for the massacres but refused to classify this action as a war crime or as an act of mass murder. The investigation was closed on the grounds the perpetrators were dead, and since the Russian government would not classify the dead as victims of the Great Purge, formal posthumous rehabilitation was deemed inapplicable.
In November 2010, the Russian State Duma approved a declaration blaming Stalin and other Soviet officials for ordering the massacre.