A Russian human rights group lost a legal battle on Wednesday to declassify documents from a probe into the 1940 Soviet massacre of Polish officers at Katyn.
The refusal to reveal why, in 2004, prosecutors dropped an investigation into the killings may further sour relations with Warsaw, chilled by a row over an airliner crash last year which killed Poland's president on his way to Katyn.
The Russian Supreme Court did not disclose the legal grounds for its rejection of the appeal by rights group Memorial.
"We expected the court to act according to the law in announcing its verdict, but unfortunately, what's happened is an insult and a bad sign," Nikita Petrov of Memorial told Reuters.
Only after the collapse of communism did Moscow stop blaming Nazi German forces for the killing of 22,000 Polish officers and others in 1940. Last year, amid a thaw in relations with its former Cold War satellite, it published documents showing dictator Josef Stalin personally signed the execution order.
It has shared much documentation with Poland, but has not satisfied Polish demands for full disclosure about the events.
Memorial says there is no legal basis to keeping secret the court documents on the 2004 ruling to close the investigation because it now contains no state secrets. The group now plans to appeal to the European Court of Human Rights.
Last April, Polish President Lech Kaczynski was leading a delegation to a 70th anniversary commemoration at Katyn for those massacred when his plane crashed. That disaster encouraged a rapprochement between Moscow and Warsaw. Recriminations over who was at fault have since poisoned the atmosphere.
This month, a Russian inquiry blamed the Polish pilots and officials aboard for pressuring them to try and land in fog. The report provoked anger in Poland. Kaczynski's brother, who leads the opposition, accuses the government of being soft on Russia.
Alexander Gurianov, the head of the Polish section of the Memorial group, said he was not surprised by the latest court decision. "They've never ruled in our favour," he said.
As well as turning to the European court at Strasbourg, he said, Memorial would also take the case to a final recourse in Moscow, by appealing to the chairman of the Supreme Court.