Katyn massacre memorial in Świętokrzyskie Mountains, Poland:
The first news of the massacre at Katyn forest came in April 1943 when the Germans found a mass grave of 4,500 Polish soldiers in German-occupied Russia. The discovery of the Katyn massacre was to greatly embarrass the Russian government.
The Soviet invasion of Poland began on 17 September in accordance with the Molotov-Ribbentrop Pact. The Red Army advanced quickly and met little resistance, as Polish forces facing them were under orders not to engage the Soviets. About 250,000to 454,700 Polish soldiers and policemen were captured and interned by the Soviet authorities. Some were freed or escaped quickly, but 125,000 were imprisoned in camps run by the NKVD. Of these, 42,400 soldiers, mostly of Ukrainian and White Russian ethnicity serving in the Polish army, who lived in the territories of Poland annexed by the Soviet Union, were released in October. The 43,000 soldiers born in western Poland, then under German control, were transferred to the Germans; in turn, the Soviets received 13,575 Polish prisoners from the Germans.
According to a report from 19 November 1939, the NKVD had about 40,000 Polish POWs: 8,000-8,500 officers and warrant officers, 6,000-6,500 officers of police, and 25,000 soldiers and non-commissioned officers who were still being held as POWs. In December, a wave of arrests resulted in the imprisonment of additional Polish officers. Ivan Serov reported to Lavrentiy Beria on 3 December that “in all, 1,057 former officers of the Polish Army had been arrested”. The 25,000 soldiers and non-commissioned officers were assigned to forced labor (road construction, heavy metallurgy).
On 5 March 1940, pursuant to a note to Joseph Stalin from Beria, six members of the Soviet Politburo — Stalin, Vyacheslav Molotov, Lazar Kaganovich, Kliment Voroshilov, Anastas Mikoyan, and Mikhail Kalinin — signed an order to execute 25,700 Polish “nationalists and counterrevolutionaries” kept at camps and prisons in the occupied western Ukraine and White Russia. The reason for the massacre, according to the historian Gerhard Weinberg, was that Stalin wanted to deprive a potential future Polish military of a large portion of its talent.
The first transport, on 4 April 1940, carried 390 people, and the executioners had difficulty killing so many people in one night. The following transports held no more than 250 people. The executions were usually performed with German-made .25 ACP Walther Model 2 pistols supplied by Moscow, but Soviet-made 7.62×38mmR Nagant M1895 revolvers were also used.The executioners used German weapons rather than the standard Soviet revolvers, as the latter were said to offer too much recoil, which made shooting painful after the first dozen executions. Vasily Mikhailovich Blokhin, chief executioner for the NKVD—and quite possibly the most prolific executioner in history—is reported to have personally shot and killed 7,000 of the condemned, some as young as 18, from the Ostashkov camp at Kalinin prison, over a period of 28 days in April 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. In addition to muffling by the rough insulation in the execution cell, the pistol gunshots were also masked by the operation of loud machines (perhaps fans) throughout the night. Some post-1991 revelations suggest that prisoners were also executed in the same manner at the NKVD headquarters in Smolensk, though judging by the way that the corpses were stacked, some captives may have been shot while standing on the edge of the mass graves. This procedure went on every night, except for the public May Day holiday.
Some 3,000 to 4,000 Polish inmates of Ukrainian prisons and those from Belarus prisons were probably buried in Bykivnia and in Kurapaty respectively, about 50 women among them. Lieutenant Janina Lewandowska, daughter of Gen. Józef Dowbor-Muśnicki, was the only woman P.O.W. executed during the Katyn massacre.
I recommend you to watch the Polish movie Katyn (Katyń). It is based on the book Post Mortem: The Story of Katyn by Andrzej Mularczyk. It was nominated for Best Foreign Language Film for the 80th Academy Awards.