The vague publicly stated reason for the crash was that there was an explosion of foreign origin. The Finnish government knew that the plane was shot down by the Soviets, but chose to withhold this information for foreign policy reasons.
The airline's CEO confided the truth to the pilots' widows, who were sworn to secrecy. The true story was kept secret for almost 50 years until a Finnish aviation historian happened to find a copy of a Soviet air force officer's memoirs which describe the incident.
https://en.wikipedia.org/wiki/Kaleva_(airplane)
https://yle.fi/aihe/artikkeli/2018/04/24/matkustajakone-kale... (in Finnish)