Katyn massacre

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 April and May 1940. Stalin ordered it on Beria's advice. 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. 
In the 1980s, Michail Gorbachev said that Beria and Vsevolod Merkulov were responsible. He said that the Katyn massacre was one of Stalin's atrocities. 
Beria was accused of treason. He was executed in 1953. Merkulov was also executed in 1953 for being close to Beria. Beria and Merkulov were never tried for what they did at Katyn.