STRASBOURG, France (Reuters) - The European Court of Human Rights ruled on Monday that Russia had violated the rights of relatives of thousands of Poles who were killed by the Soviet secret police in 1940, and described the Katyn massacre as a "war crime".
The ruling followed a complaint by 15 descendants of 12 victims over the adequacy of Russia's enquiry into the massacre, in which some 22,000 Polish officers and intellectuals were murdered without trial, often by shooting them from behind.
"(The applicants) suffered a double trauma: losing their relatives in the war and not being allowed to learn the truth about their death for more than 50 years," the European Court of Human Rights (ECHR) said in a statement.
The document added: "It (the court) found that the mass murder of the Polish prisoners by the Soviet secret police had been a war crime."
After initially blaming the killings on Poland's wartime Nazi occupiers, the Soviet Union accepted responsibility in 1990, beginning an investigation that was abandoned in 2004.
In recent years, Russia has released some documents regarding the massacre and the later investigation, but others still remain classified and inaccessible to the Polish side.
The 1940 Katyn massacre remains a stumbling block in bilateral ties between Warsaw and Moscow.
Poland, which has campaigned for the release of full lists of victims and their legal rehabilitation, welcomed the ruling.
"It is an important fact that the Katyn massacre is now recognized as a war crime in the understanding of international law," said Marcin Bosacki, a spokesman for the Polish Foreign Ministry.
The Strasbourg-based court said Russia had breached Article 3 of the EU Human Rights Convention prohibiting inhumane treatment by failing to provide 10 of the relatives with details of the deaths and of its investigation into the massacre.
It said Russia had also violated its obligation to cooperate with the court by refusing to submit information needed to examine the case.
"The approach chosen by the Russian military courts to maintain to the applicants' face... that their relatives had somehow vanished in the Soviet camps demonstrated a callous disregard for the applicants' concerns," the ruling said.
No damages were awarded to the victims, but Russia was ordered to pay the applicants 5,000 euros ($6,500) for costs.
The Court said only 10 of the plaintiffs - a widow and nine children of the victims - could be said to have received inhumane treatment, as the others had never had contact with their missing fathers so their suffering could not be assessed.
It also said it could not rule on the merits of the Soviet investigation as it related to events that had taken place before Russia ratified the Human Rights Convention in 1998.
Andrei Fedorov, Moscow's envoy to Strasbourg, called the result a positive outcome for his country.
"The decision on the main point - that it is outside of the court's competence to see grounds to rule on reopening of the investigation into the Katyn case - is in Russia's favor," he told Interfax.
Poland's Bosacki said the victims' families would probably appeal the ruling on the investigation. Russia also has the right to appeal the ruling on its violation of Article 3.
In 2010, Poland's then-president Lech Kaczynski and 95 other people died in a plane crash in western Russia while travelling to the Katyn Forest to honor the 70th anniversary of the massacre.
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(Reporting By Gilbert Reilhac in Strasbourg; Writing by Vicky Buffery Editing by Maria Golovnina)