The Katyn massacre was a series of mass executions of nearly 22,000 Polish military officers and intelligentsia prisoners of war carried out by the Soviet Union, specifically the NKVD in April and May 1940. The massacre is named after the Katyn Forest, where some of the mass graves were first discovered by German forces.
The Soviet Union 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 that 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.
Documents released lend weight to the belief that suppression within the highest levels of the U.S. government helped cover up Soviet guilt in the killing of some 22,000 Polish officers and other prisoners in the Katyn forest and other locations in 1940.
The most dramatic revelation is the evidence of the secret codes sent by the two American POWS - something historians were unaware of and which adds to evidence that the Roosevelt administration knew of the Soviet atrocity relatively early on.
The reason for the massacre, according to the historian Gerhard Weinberg, was that Stalin wanted to deprive a potential future Polish military of a large portion of its talent.