Prosecutors on Thursday turned over to Poland 20 new files from a probe into the 1940 Katyn massacre that could be key in proving that Soviet secret police carefully planned the killing of thousands of Poles.
Saak Karapetyan, chief of the international legal department at the Prosecutor General's Office, handed two boxes to Polish diplomat Piotr Marciniak during a ceremony at the prosecutors' headquarters in Moscow.
The World War II massacre of some 20,000 Polish officers and other prominent citizens in western Russia by Soviet secret police has long soured relations between the countries, which were also burdened by other historic events.
The new files could be key in uncovering the role of the Soviet secret service in the massacre. The files contain the full list of the Polish prisoners of war executed in the Katyn forest, Russian secret police documents confirming their dispatch there as well as interrogation protocols and files confirming the burial place of the Poles, Karapetyan said.
The Nazis discovered the mass graves during their march on Moscow in the fall of 1941, but Soviet propaganda blamed the deaths on Adolf Hitler and punished anyone speaking the truth with harsh prison terms.
In 1990, Moscow acknowledged that dictator Josef Stalin's secret police were responsible. But Russian officials refused to refer to Katyn as a genocide attempt — a designation that Poland had sought because international law generally considers that genocide has no statute of limitations.
Russia began a criminal investigation the same year, but closed it in 2004 because the killings were found not to be genocide.
President Dmitry Medvedev turned over 67 volumes to acting Polish President Bronislaw Komorowski in May in an effort to mend ties with its western neighbor. Komorowski at the time described Katyn as "an ordeal experienced jointly by both Poland and Russia."
Karapetyan said Russia is "continuing its work to fulfill Poland's request" to provide more files.
Polish envoy Marciniak on Thursday expressed gratitude and said he hoped that Russia and Poland's joint efforts will help "establish the whole truth about Katyn."
Speaking to reporters after the ceremony, the Polish diplomat said Russia's move proves that Moscow is no longer hiding evidence about Katyn. "Before that, we were under the impression that these 20 files were supposed to remain secret," he said.
Katyn inadvertently drew international attention in April when Polish President Lech Kaczynski, his wife and 94 other Poles heading for a Katyn commemoration died in a plane crash in western Russia.
The death of the Polish leader on Russian land provided the two neighboring nations with a rare chance to move forward and leave old grudges behind.
Tsarist Russia ruled much of Poland for over two centuries and ruthlessly crushed popular revolts. The 1920 Bolshevik attempt to regain control over Poland ended with a humiliating defeat.
Some Russian historians claim that Polish authorities massacred thousands of captive Red Army soldiers and allege that Katyn was Stalin's response to the defeat.