The term "Katyn massacre" originally referred specifically to the massacre at Katyn Forest, near the villages of Katyn and Gnezdovo (approximately 19 kilometers [12 miles] west of Smolensk, Russia), of Polish military officers in the Kozelsk prisoner-of-war camp.

This was the largest of several concurrent executions of prisoners of war. Other executions were carried out at the geographically distant Starobelsk and Ostashkov camps, at the NKVD headquarters in Smolensk, and at prisons in Kalinin (Tver), Kharkiv, Moscow, and other Soviet cities. Still more executions took place at various locations in Belarus and Western Ukraine, based on special lists of Polish prisoners prepared by the NKVD specifically for those regions. The investigations of the killings made in the 1990s covered not only the massacre at Katyn forest, but also the other mass murders mentioned above. Polish organisations such as the Katyn Committee and the Federation of Katyn Families consider the victims executed at the locations other than Katyn to be victims of the larger coordinated operation.

The government of Nazi Germany announced the discovery of mass graves in the Katyn Forest in 1943. When the London-based Polish government-in-exile asked for an investigation by the International Red Cross, Stalin immediately severed diplomatic relations with it. The Soviet Union claimed the victims had been murdered by the Nazis, and 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.

An investigation conducted by the Prosecutor General's Office of the Soviet Union (1990–1991) and the Russian Federation (1991–2004), confirmed Soviet responsibility for the massacres. It was able to confirm the deaths of 1,803 Polish citizens but refused to classify this action as a war crime or an act of genocide. The investigation was closed on the grounds that the perpetrators of the massacre were already dead, and since the Russian government would not classify the dead as victims of Stalinist repression, formal posthumous rehabilitation was deemed inapplicable. The human rights society Memorial issued a statement which declared "this termination of investigation is inadmissible" and that the confirmation of only 1,803 people killed "requires explanation because it is common knowledge that more than 14,500 prisoners were killed". In November 2010, the Russian State Duma approved a declaration blaming Stalin and other Soviet officials for having personally ordered the massacre.

The number of victims is estimated at about 22,000, with a lower limit of confirmed dead of 21,768. According to Soviet documents declassified in 1990, 21,857 Polish internees and prisoners were executed after 3 April 1940: 14,552 prisoners of war (most or all of them from the three camps) and 7,305 prisoners in western parts of the Byelorussian and Ukrainian SSRs. Of them 4,421 were from Kozelsk, 3,820 from Starobelsk, 6,311 from Ostashkov, and 7,305 from Byelorussian and Ukrainian prisons. Head of the NKVD POW department, Maj. General P. K. Soprunenko, organized "selections" of Polish officers to be massacred at Katyn and elsewhere.

Those who died at Katyn included an admiral, two generals, 24 colonels, 79 lieutenant colonels, 258 majors, 654 captains, 17 naval captains, 3,420 Non-commissioned officers, seven chaplains, three landowners, a prince, 43 officials, 85 privates, 131 refugees, 20 university professors, 300 physicians; several hundred lawyers, engineers, and teachers; and more than 100 writers and journalists as well as about 200 pilots. In all, the NKVD executed almost half the Polish officer corps. Altogether, during the massacre the NKVD executed 14 Polish generals: Leon Billewicz (ret.), Bronisław Bohatyrewicz (ret.), Xawery Czernicki (admiral), Stanisław Haller (ret.), Aleksander Kowalewski (ret.), Henryk Minkiewicz (ret.), Kazimierz Orlik-Łukoski, Konstanty Plisowski (ret.), Rudolf Prich (murdered in Lviv), Franciszek Sikorski (ret.), Leonard Skierski (ret.), Piotr Skuratowicz, Mieczysław Smorawiński and Alojzy Wir-Konas (promoted posthumously). Not all of the executed were ethnic Poles, because the Second Polish Republic was a multiethnic state, and its officer corps included Belorussians, Ukrainians, and Jews. It is estimated that about 8% of Katyn massacre victims were Polish Jews. 395 prisoners were spared from the slaughter, among them Stanisław Swianiewicz and Józef Czapski. They were taken to the Yukhnov camp and then to Gryazovets.

Up to 99% of the remaining prisoners were subsequently murdered. People from the Kozelsk camp were executed in the Katyn forest; people from the Starobelsk camp were murdered in the inner NKVD prison of Kharkiv and the bodies were buried near the village of Piatykhatky; and police officers from the Ostashkov camp were murdered in the internal NKVD prison of Kalinin (Tver) and buried in Mednoye.

Detailed information on the executions in the Kalinin NKVD prison was provided during a hearing by Dmitrii Tokarev, former head of the Board of the District NKVD in Kalinin. According to Tokarev, the shooting started in the evening and ended at dawn. 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 7.65×17mm Walther PPK pistols supplied by Moscow, but 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. Vasili 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 personal information of the condemned was checked, he was handcuffed and led to a cell insulated with stacks of sandbags along the walls and a felt-lined, heavy door. The victim was told to kneel in the middle of the cell, was then approached from behind by the executioner and immediately shot in the back of the head. 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 subject 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. This procedure went on every night, except for the 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. Lieutenant Janina Lewandowska, daughter of Gen. Józef Dowbor-Muśnicki, was the only woman executed during the massacre at Katyn.

The question about the fate of the Polish prisoners was raised soon after the Axis invasion of the Soviet Union in June 1941. The Polish government-in-exile and the Soviet government signed the Sikorski-Mayski Agreement which announced the willingness of both to fight together against Nazi Germany and for a Polish army to be formed on Soviet territory. The Polish general Władysław Anders began organizing this army, and soon he requested information about the missing Polish officers. During a personal meeting, Stalin assured him and Władysław Sikorski, the Polish Prime Minister, that all the Poles were freed, and that not all could be accounted because the Soviets "lost track" of them in Manchuria.

In 1942, with the territory around Smolensk under German occupation, captive Polish railroad workers heard from the locals about a mass grave of Polish soldiers at Kozelsk near Katyn, found one of the graves, and reported it to the Polish Secret State. The discovery was not seen as important, as nobody thought that the discovered grave could contain so many victims. In early 1943, Rudolf von Gersdorff, a German officer serving as the intelligence liaison between the Wehrmacht's Army Group Center and Abwehr, received reports about mass graves of Polish military officers. These reports stated the graves were in the forest of Goat Hill near Katyn. He passed the reports to his superiors (sources vary on when exactly the Germans became aware of the graves — from "late 1942" to January–February 1943, and when the German top decision makers in Berlin received those reports [as early as 1 March or as late as 4 April]). Joseph Goebbels saw this discovery as an excellent tool to drive a wedge between Poland, Western Allies, and the Soviet Union, and reinforce the Nazi propaganda line about the horrors of Bolshevism and American and British subservience to it. After extensive preparation, on 13 April, Berlin Radio broadcast to the world that German military forces in the Katyn forest near Smolensk had uncovered a ditch that was "28 metres long and 16 metres wide [92 ft by 52 ft], in which the bodies of 3,000 Polish officers were piled up in 12 layers". The broadcast went on to charge the Soviets with carrying out the massacre in 1940.

The Germans brought in a European Red Cross committee called the Katyn Commission consisting of twelve forensic experts and their staff from Belgium, Bulgaria, Denmark, Finland, France, Italy, Croatia, the Netherlands, Romania, Sweden, Slovakia, and Hungary. They were so intent on proving that the Soviets were behind the massacre that they even included some Allied prisoners of war, among them writer Ferdynand Goetel, the Polish AK prisoner from Pawiak. After the war, Goetel escaped with a fake passport due to an arrest warrant issued against him; two of the twelve, the Bulgarian, Marko Markov and the Czech, Frantisek Hajek, with their countries becoming satellite states of the Soviet Union, were forced to recant their evidence, defending the Soviets and blaming the Germans. Pathologist Ljudevit Jurak, member of investigation team from Croatia, was sentenced to death and executed by communists after the war, while his colleague Eduard Miloslavić managed to escape to USA. The Katyn massacre was beneficial to Nazi Germany, which used it to discredit the Soviet Union. On 14 April 1943 Goebbels wrote in his diary: "We are now using the discovery of 12,000 Polish officers, murdered by the GPU, for anti-Bolshevik propaganda on a grand style. We sent neutral journalists and Polish intellectuals to the spot where they were found. Their reports now reaching us from ahead are gruesome. The Führer has also given permission for us to hand out a drastic news item to the German press. I gave instructions to make the widest possible use of the propaganda material. We shall be able to live on it for a couple weeks". The Germans won a major propaganda victory, portraying communism as a danger to Western civilization.

The Soviet government immediately denied the German charges and claimed that the Polish prisoners of war had been engaged in construction work west of Smolensk and consequently were captured and executed by invading German units in August 1941. The Soviet response on 15 April to the initial German broadcast of 13 April, prepared by the Soviet Information Bureau, stated that "Polish prisoners-of-war who in 1941 were engaged in construction work west of Smolensk and who...fell into the hands of the German-Fascist hangmen".

In April 1943 the Polish government-in-exile insisted on bringing the matter to the negotiation table with the Soviets and on opening an investigation by the International Red Cross. Stalin, in response, accused the Polish government of collaborating with Nazi Germany, broke off diplomatic relations with it, and started a campaign to get the Western Allies to recognize the alternative Polish pro-Soviet government in Moscow led by Wanda Wasilewska. Sikorski died in an air crash in July—an event that was convenient for the Allied leaders.

Soviet actions

When, in September 1943, Goebbels was informed that the German army had to withdraw from the Katyn area, he wrote a prediction in his diary. His entry for 29 September 1943 reads: "Unfortunately we have had to give up Katyn. The Bolsheviks undoubtedly will soon 'find' that we shot 12,000 Polish officers. That episode is one that is going to cause us quite a little trouble in the future. The Soviets are undoubtedly going to make it their business to discover as many mass graves as possible and then blame it on us".

Having retaken the Katyn area almost immediately after the Red Army had recaptured Smolensk, around September–October 1943, NKVD forces began a cover-up operation. A cemetery the Germans had permitted the Polish Red Cross to build was destroyed and other evidence removed. Witnesses were "interviewed", and threatened with arrest for collaborating with the Germans if their testimonies disagreed with the official line. As none of the documents found on the dead had dates later than April 1940 the Soviet secret police planted false evidence to push the apparent time of the massacre back to the summer of 1941, when the Germans controlled the area. A preliminary report was issued by NKVD operatives Vsevolod Merkulov and Sergei Kruglov, dated 10–11 January 1944, concluding that the Polish officers were shot by the Germans.

In January 1944, the Soviet Union sent another commission, the Special Commission for Determination and Investigation of the Shooting of Polish Prisoners of War by German-Fascist Invaders in Katyn Forest (Russian: Специальная Комиссия по установлению и расследованию обстоятельств расстрела немецко-фашистскими захватчиками в Катынском лесу военнопленных польских офицеров, Spetsial'naya Kommissiya po ustanovleniyu i rassledovaniyu obstoyatel'stv rasstrela nemetsko-fashistskimi zakhvatchikami v Katynskom lesu voyennoplennyh polskih ofitserov) to the site; the very name of the commission implied a predestined conclusion. It was headed by Nikolai Burdenko, the President of the Academy of Medical Sciences of the USSR (hence the commission is often known as the "Burdenko Commission"), who was appointed by Moscow, to investigate the incident. Its members included prominent Soviet figures such as the writer Alexei Tolstoy, but no foreign personnel were allowed to join the Commission. The Burdenko Commission exhumed the bodies, rejected the 1943 German findings that the Poles were shot by the Soviets, assigned the guilt to the Germans and concluded that all the shootings were done by German occupation forces in autumn of 1941. Despite lack of evidence, it also blamed the Germans for shooting Russian prisoners of war used as labor to dig the pits. It is uncertain how many members of the commission were misled by the falsified reports and evidence, and how many suspected the truth; Cienciala and Materski note that the Commission had no choice but to issue findings in line with the Merkulov-Kruglov report, and that Burdenko himself likely was aware of the cover-up. He reportedly admitted something like that to friends and family shortly before his death. The Burdenko commission's conclusions would be consistently cited by Soviet sources until the official admission of guilt by the Soviet government on 13 April 1990.

In January 1944, the Soviets also invited a group of more than a dozen mostly American and British journalists, accompanied by Kathleen Harriman, the daughter of the new American ambassador W. Averell Harriman), and John Melby, third secretary at the American embassy in Moscow, to Katyn. That Melby and Harriman were included was regarded by some at the time as an attempt by the Soviets to lend official weight to their propaganda. Melby's report noted the deficiencies in the Soviet case: problematic witnesses; attempts to question the witnesses were discouraged; statements by witnesses were obviously given as a result of rote memorization and "the show was put on for the benefit of the correspondents". Nevertheless Melby, at the time, felt that on balance the Russian case was convincing. Harriman's report reached the same conclusion and both were asked to explain after the war why their conclusions seemed to be at odds with their findings, with the suspicion that the conclusions were those the State Department wanted to hear. The journalists were less impressed, and not totally convinced by the staged Soviet demonstration.