Russian prosecutors will hand over to Poland on Thursday another batch of documents on the Katyn massacre following a request by Polish investigators.
The Katyn massacre, in which thousands of officers, police and civilians taken prisoner during the 1939 partitioning of Poland by the Soviet Union and Nazi Germany were killed, remains one of the most painful issues in Russian-Polish relations.
Last year, Russia handed over to Poland hundreds of files of its investigation into the killing of over 21,000 Poles in 1940.
The files contained additional lists of Polish servicemen held captive by the Soviet secret police, interrogation and forensic reports, medical records, burial certificates and other data related to the massacre.
The Soviet Union always blamed the massacre on the Nazis, saying the killings took place in 1941, when the territory was in German hands.
However, Soviet leader Mikhail Gorbachev formally admitted in 1990 that the executions took place around 1940, and were carried out by the Soviet secret police, the NKVD.
In the 1990s, Russia handed over to Poland copies of archive documents from the top-secret File No.1, which placed the blame solely on the Soviet Union.
In September 1990, Russian prosecutors also launched a criminal case into the massacre, known as "Case No.159." The investigation was closed in 2004.
Poland's Institute of National Remembrance, which has been investigating the case since 2004, has proposed including Russia's materials into its own investigation.
In November 2010, lawmakers from the lower house of Russia's parliament approved a declaration recognizing the Katyn massacre as a crime committed by Joseph Stalin's regime.
Poland has hailed the recognition of the massacre.
MOSCOW, April 7 (RIA Novosti)
