Short answers, pulled from the story.
The Katyn massacre was ordered by Joseph Stalin and the Soviet Politburo. On the 5th of March 1940, six Politburo members including Stalin, Vyacheslav Molotov, Lazar Kaganovich, Kliment Voroshilov, Anastas Mikoyan, and Mikhail Kalinin signed an order to execute 25,700 Polish prisoners. The order was issued on the basis of a proposal from NKVD head Lavrentiy Beria.
Nearly 22,000 people were killed in the Katyn massacre. Soviet documents declassified in 1990 confirmed 21,857 Polish internees and prisoners were executed after the 3rd of April 1940. The victims included military officers, police officers, border guards, intelligentsia, university professors, physicians, lawyers, and journalists.
The Soviet Union officially admitted responsibility on the 13th of April 1990, the forty-seventh anniversary of the German announcement of the mass graves. The USSR formally expressed "profound regret" and acknowledged that the NKVD carried out the killings. The day was subsequently declared a worldwide Katyn Memorial Day.
The killings took place at multiple sites. Prisoners from the Kozelsk camp were executed in the Katyn Forest near Smolensk. Those from the Starobelsk camp were killed in the NKVD prison in Kharkiv and buried near the village of Piatykhatky. Police officers from Ostashkov were killed in the NKVD prison in Kalinin (now Tver) and buried at Mednoye. All three burial sites had previously served as secret cemeteries for victims of the Great Purge.
After retaking the Katyn area in September-October 1943, the NKVD destroyed a Polish Red Cross cemetery, planted false evidence to suggest the killings occurred in 1941 when Germany controlled the area, and threatened witnesses with arrest for Nazi collaboration if their accounts deviated from the official line. The Burdenko Commission, led by Nikolai Burdenko of the USSR Academy of Medical Sciences, was convened to officially attribute the crime to German forces. In the 1950s, KGB chief Alexander Shelepin proposed and oversaw the destruction of many documents related to the massacre.
On the 10th of April 2010, an aircraft carrying Polish president Lech Kaczynski, his wife, and 87 other senior Polish officials crashed near Smolensk, killing all 96 aboard. The delegation had been travelling to attend a ceremony marking the 70th anniversary of the Katyn massacre. Prime Minister Donald Tusk described it as "the most tragic Polish event since the war."