The Katyn massacre is the name of a series of killings by the Soviet army during World War II. Members of the NKVD killed about 22,000 Polish prisoners of war. In a forest near Katyn, about from Smolensk, most of the prisoners killed were officers. The Katyn massacre happened in May 1940. Stalin ordered it on Beria's advice. Between 4,000 and 5,000 people were killed in Katyn. In 1990, the Soviet Union/Russia admitted it was responsible. In 1990, Mikhail Gorbachev officially apologised. The Katyn massacre is one in a series of similar massacres where up to 100,000 people were killed. Between 400 and 450 prisoners survived. People used their stories to change Soviet history books after the apology.