Task: Question Answering
Input (English): Passage: Kozelsk and Starobelsk were used mainly for military officers, while Ostashkov was used mainly for Polish Scouting, gendarmes, police officers, and prison officers. Some prisoners were members of other groups of Polish intelligentsia, such as priests, landowners, and law personnel. The approximate distribution of men throughout the camps was as follows: Kozelsk, 5000; Ostashkov, 6570; and Starobelsk, 4000. They totalled 15,570 men.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).Once at the camps, from October 1939 to February 1940, the Poles were subjected to lengthy interrogations and constant political agitation by NKVD officers, such as Vasily Zarubin. The prisoners assumed they would be released soon, but the interviews were in effect a selection process to determine who would live and who would die. According to NKVD reports, if a prisoner could not be induced to adopt a pro-Soviet attitude, he was declared a "hardened and uncompromising enemy of Soviet authority".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 occupied western Ukraine and Belarus. 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:
It has been suggested that the motive for this terrible step [the Katyn massacre] was to reassure the Germans as to the reality of Soviet anti-Polish policy. This explanation is completely unconvincing in view of the care with which the Soviet regime kept the massacre secret from the very German government it was supposed to impress. […] A more likely explanation is that [the massacre] should be seen as looking forward to a future in which there might again be a Poland on the Soviet Union's western border. Since he intended to keep the eastern portion of the country in any case, Stalin could be certain that any revived Poland would be unfriendly. Under those circumstances, depriving it of a large proportion of its military and technical elite would make it weaker. 
Question: What is the name of the massacre kept secret by the Soviet regime from the very German government it was supposed to impress?
Output (English): Katyn massacre.