Russia has published online once-secret files on the 1940 Katyn massacre, in which some 22,000 members of the Polish elite were killed by Soviet forces.
The state archive said the "Packet No. 1" original files had until now only been available to researchers.
The Soviet Union denied its role in the massacre for decades.
But relations between Russia and Poland have warmed since the Polish president and others were killed in a plane crash on their way to a Katyn commemoration.
The six documents that were published on the state archive website were declassified in 1992 on the order of the then-Russian president, Boris Yeltsin.
Current President Dmitry Medvedev had now ordered their publication online, the state archive said.
One of the documents is a 5 March, 1940 letter from the then-head of the Soviet secret police or NKVD, Lavrenty Beria, to Soviet leader Joseph Stalin, recommending the execution of Polish prisoners of war.
Beria refers to them as "steadfast, incorrigible enemies of Soviet power".
"Each of them is just waiting for liberation so as to actively join the struggle against Soviet power," it says.
The letter bears Stalin's signature in blue pencil, with the comment "In favour".
Given that historians have already had access to the files for some time, correspondents say the decision to put them on the state archive website is likely to be seen as a symbolic gesture, rather than shedding new light on what happened at Katyn.
"We on the Russian side are showing absolute openness in telling what happened in Katyn and other places with Polish prisoners of war," Russian state archive chief Andrei Artyzov was quoted as saying by Russian news agencies.
"All the basic documents about these events have been published."
Among the files that remain secret are documents relating to a Russian investigation into the massacre that began in the 1990s.
Russian human rights campaigners have appealed for those documents to be declassified.
Poland has repeatedly demanded that Russia open all its files on Katyn, and the issue has soured relations between the two countries in the past.
Recently though, tension over Katyn has eased.
Earlier this month leaders from both states marked the massacre together for the first time, in a joint ceremony attended by Russian Prime Minister Vladimir Putin and his Polish counterpart Donald Tusk.
It was the first Russian ceremony to commemorate Katyn.
Days later, Polish President Lech Kaczynski and more than 90 others were killed when their plane crashed as it was trying to land in western Russia ahead of a separate event to mark the killings.
Moscow's handling of the aftermath of the crash was well received by Poles.
The April 1940 killings were carried out by the NKVD on Stalin's orders.
Members of the Polish elite, including officers, politicians and artists, were shot in the back of the head and their bodies dumped in mass graves.
The killings took place at various sites, but the western Russian forest of Katyn has become their chief symbol.
The Soviet Union blamed the massacre on Nazi Germany before acknowledging responsibility in 1990.
One of the documents now posted online was a March 1959 letter marked "Top Secret" from the former head of the KGB, Alexander Shelepin, to then-Soviet leader Nikita Khrushchev, proposing that all dossiers concerning the Katyn killings be destroyed.
He said the authorities should just keep a few documents - the minutes of meetings of the NKVD troika that condemned the prisoners and some papers on the fulfilment of the troika's instructions.
Shelepin wrote that the official Soviet version - that Nazi Germany had carried out the killings - had been "firmly implanted in international opinion".