Katyn massacre was a shooting the spring of 1940 at least 21 768 citizens of Polish , including over 10 thousand. officers of the army and the police, by a decision of the highest authorities of the Union of Soviet Socialist Republics. Execution of the victims, identified as "enemies of the Soviet power" and killed by shots in the back of the head with a handgun , made the Soviet political police, the NKVD . In the years 1940-1990 the Soviet authorities denied its responsibility for the Katyn massacre, but the April 13, 1990, they officially admitted that it was "one of the serious crimes of Stalinism ". The victims of the crime were officers, noncommissioned officers and a number of the Polish Army , partly derived from the reserve (scientists, doctors, engineers, lawyers, teachers, civil servants, entrepreneurs, members of the professions)