Or until they say the examiners set up the bodies themselves. The most common response of Kremlin is attacking the attacker with the same words, saying "it wasn't us, it was you".

The massacre was initiated in NKVD chief Lavrentiy Beria's proposal to Joseph Stalin to execute all captive members of the Polish officer corps, which was approved by the Soviet Politburo led by Stalin.[1] Of the total killed, about 8,000 were officers imprisoned during the 1939 Soviet invasion of Poland, another 6,000 were police officers, and the remaining 8,000 were Polish intelligentsia the Soviets deemed to be intelligence agents and gendarmes, spies and saboteurs, former landowners, factory owners and officials "intelligence agents and gendarmes, spies and saboteurs, former landowners, factory owners and officials".[2] The Polish Army officer class was representative of the multi-ethnic Polish state; the murdered included ethnic Poles, Ukrainians, Belarusians, and Jews including the chief Rabbi of the Polish Army, Baruch Steinberg.[3]

The government of Nazi Germany announced the discovery of mass graves in the Katyn Forest in April 1943
The USSR claimed the Nazis had killed the victims, and it continued to deny responsibility for the massacres until 1990, when it officially acknowledged and condemned the killings by the NKVD, as well as the subsequent cover-up by the Soviet government.
https://en.wikipedia.org/wiki/Katyn_massacre