The Katyn massacre, also known as the Katyn Forest massacre (Polish: zbrodnia katyńska, mord katyński, 'Katyń crime'; Russian: Катынский расстрел), was a mass murder of Polish nationals carried out by the Soviet secret police NKVD in April--May 1940. It was based on Lavrentiy Beria's proposal to execute all members of the Polish Officer Corps, dated 5 March 1940. This official document was then approved and signed by the Soviet Politburo, including its leader, Joseph Stalin. The number of victims is estimated at about 22,000, the most commonly cited number being 21,768. The victims were murdered in the Katyn Forest in Russia, the Kalinin and Kharkov prisons and elsewhere. About 8,000 were officers taken prisoner during the 1939 Soviet invasion of Poland, the rest being Polish doctors, professors, lawmakers, police officers, and other public servants arrested for allegedly being "intelligence agents, gendarmes, landowners, saboteurs, factory owners, lawyers, officials and priests." Since Poland's conscription system required every unexempted university graduate to become a reserve officer, the NKVD was able to round up much of the Polish intelligentsia, and the Russian, Ukrainian, Tatar, Jewish, Georgian, and Belarusian intelligentsia of Polish citizenship.
The term "Katyn massacre" originally referred specifically to the massacre at Katyn Forest, near the villages of Katyn and Gnezdovo (ca. 19 kilometres (12 mi) west of Smolensk, Russia), of Polish military officers in the Kozelsk prisoner-of-war camp. This was the largest of several roughly simultaneous executions of prisoners of war; the others included executions at the geographically distant Starobelsk and Ostashkov camps, and the executions of political prisoners from West Belarus and West Ukraine, shot at Katyn Forest, at the NKVD headquarters in Smolensk, at a Smolensk slaughterhouse, and at prisons in Kalinin (Tver), Kharkov, Moscow, and other Soviet cities. The Belorussian and Ukrainian Katyn Lists are NKVD lists of names of Polish prisoners to be murdered at various locations in Belarus and Western Ukraine. The modern Polish investigation of the Katyn massacre covered not only the massacre at Katyn forest, but also the other mass murders mentioned above. There are Polish organisations such as the Katyn Committee and the Federation of Katyn Families, which again are inclusive of victims of the various mass murders at the various locations.
Nazi Germany announced the discovery of mass graves in the Katyn Forest in 1943. The revelation led to the end of diplomatic relations between Moscow and the London-based Polish government-in-exile. The Soviet Union continued to deny responsibility for the massacres until 1990, when it officially acknowledged and condemned the perpetration of the killings by the NKVD, as well as the subsequent cover-up.
An investigation conducted by the Prosecutor General's Office of the Soviet Union (1990--1991) and the Russian Federation (1991--2004), has confirmed Soviet responsibility for the massacres. It was able to confirm the deaths of 1,803 Polish citizens but refused to classify this action as a war crime or an act of genocide. The investigation was closed on grounds that the perpetrators of the massacre were already dead, and since the Russian government would not classify the dead as victims of Stalinist repression, formal posthumous rehabilitation was ruled out. The human rights society Memorial issued a statement which declared "this termination of investigation is inadmissible" and that their confirmation of only 1,803 people killed "requires explanation because it is common knowledge that more than 14,500 prisoners were killed."
In November 2010, the Russian State Duma approved a declaration blaming Stalin and other Soviet officials for having personally ordered the massacre.