MOSCOW (AP) – Russia 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 Russian 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.