The killings have been dubbed “the Little Katyn,” in reference to the 22,000 Poles shot in 1940 by the Soviet NKVD secret police.
The current breakthrough was announced by Poland's state-sponsored Institute of National Remembrance (IPN), the body charged with investigating crimes against Polish citizens.
“This is proof that this murder of Poles was a crime against humanity,” said IPN chief Lukasz Kaminski on Tuesday at a press conference in Warsaw, as quoted by the Polish Press Agency.
Over 7000 people were detained in the vicinity of the Augustow Forest, north east Poland in July 1945. Some 562 were never seen again.
IPN has described the affair as “the largest crime committed by the Soviets on Polish lands after World War II.”
The missing people were linked to Poland's official wartime resistance forces, the Home Army.
The Polish government-in-exile in London had sanctioned the disbandment of the Home Army in January 1945, so as to avoid conflict with the Soviets, who were technically “liberating” Poland from the Nazis.
One of the documents obtained by IPN is a classified cipher written by General Viktor Abakamov, who in 1945 was the head of SMERSH, the Red Army's counter-intelligence agency.
The document, sent to Major General Gorgonov, calls for a plane to be sent to Olecko “"with a group of experienced counter-intelligence officers, to carry out the execution of bandits arrested in the Augustow forests.”
IPN obtained the documents from leading Russian human rights association Memorial. They were originally found by Russian historian Nikita Petrov in the archives of Russia's Federal Security Service.
IPN's announcement comes just one day after the European Court of Human Rights delivered a verdict regarding the wartime the Russian government's post 1990's investigation into the Katyn Crime.
The Strasbourg court noted that bereaved relatives of the victims “had suffered a double trauma: losing their relatives in the war and not being allowed to learn the truth about their death for more than 50 years because of the distortion of historical facts by the Soviet and Polish communist authorities.” (nh/pg)