How many Soviet war dead were shot in the back by their own officers? How many died in purges? How many in reprisals from Ukrainians? …
The Katyn Forest Massacre (inside the U.S.S.R.) was a series of mass executions of ~22,000 Polish military officers and intelligentsia carried out by the Soviet Union, specifically the NKVD ("People's Commissariat for Internal Affairs") in April and May 1940.
The killings were methodical. After the condemned individual's personal information was checked and approved, he was handcuffed and led to a cell insulated with stacks of sandbags along the walls, and a heavy, felt-lined door. The victim was told to kneel in the middle of the cell, and was then approached from behind by the executioner and immediately shot in the back of the head or neck. The body was carried out through the opposite door and laid in one of the five or six waiting trucks, whereupon the next condemned was taken inside and subjected to the same fate.
The government of Nazi Germany announced the discovery of mass graves in the Katyn Forest in April 1943. When the London-based Polish government-in-exile asked for an investigation by the International Committee of the Red Cross, Stalin immediately severed diplomatic relations with it. The USSR claimed the Nazis had killed the victims in 1941 and it continued to deny responsibility for the massacres until 1990, when it officially acknowledged and condemned the perpetration of the killings by the NKVD, as well as the subsequent cover-up by the Soviet government.