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.

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. They destroyed a cemetery the Germans had permitted the Polish Red Cross to build and removed other evidence. Witnesses were "interviewed" and threatened with arrest for collaborating with the Nazis 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 place the apparent time of the massacre in mid-1941, when the German military had controlled the area.
Conclusion: As shown in claims and counterclaims the past knowledge can be easily manipulated by many different methods and may not have the highest level of reliability, but all the knowledge humanity is using nowadays came from and is based on previous knowledge acquired by us. There are certain limitations to it but overall we should continue questioning knowledge that seems unreliable and try to avoid commonly used biased interpretations of those. Sometimes the production of the knowledge was not clearly revealed to the public due to reasons why each government has secret agencies and state secrets.