The Polish prime minister will attend Wednesday's ceremony in Russia marking the 70th anniversary of the Katyn massacre of Poles by Soviet forces. It is an unprecedented step, and one which could herald a new era in strained relations between Poland and Russia, says the BBC's Adam Easton in Warsaw.

The families of the victims of the Katyn massacre have endured decades of lies, discrimination and frustration.

For 50 years, the Soviet Union blamed the murder of more than 20,000 Polish officers on the Nazis, who uncovered one of the mass graves in the forest of Katyn, near the city of Smolensk, in 1943.

It was only in 1990 that Mikhail Gorbachev admitted Soviet responsibility.

Jolanta Klimowicz-Osmanczyk was nine when her father, Olgierd, was captured.

During the war, the family received no news of his whereabouts and afterwards the newly-installed communist government forbade any discussion of the issue.

No questions

"I lost my father, a person whom I loved very much, deeply. I am feeling even now, and maybe even deeper, the lack of a father in the whole of my life," Mrs Klimowicz-Osmanczyk told the BBC.

"You know, the most painful thing was that it was impossible to search and to talk about the case because it was absolutely out of the question.

"You know, the problem just didn't exist," she added.

Even Poland's wartime allies, Britain and the US, had not questioned the official Soviet version for fear of angering the USSR, which had become an ally in the fight against Germany.

Back in September 1939, however, at the outbreak of war, Stalin's intentions were very different - the invasion and annexation of eastern Poland, following a secret deal with Germany to carve up the country.

Approximately 230,000 Polish soldiers were taken prisoner after the Soviet invasion.

In October, Moscow decided to place and interrogate politically active and patriotic prisoners in camps run by the Soviet secret police, the NKVD.

The prisoners were led to believe they would be released but the interrogations were really a way to determine who would die.

Mass grave

Among them were generals, college professors, teachers, diplomats, civil servants, engineers, writers and artists, politicians; members of the Polish elite.

Many had taken part in Poland's military defeat of the Red Army in 1920, a campaign Stalin took part in.

In March 1940, the central committee of the Communist party accepted a proposal to kill those prisoners it classified as enemies of the state.

"Current POWs are trying to continue their counter revolutionary and anti-Soviet activities. All are awaiting their freedom to actively participate in a fight against the Soviet government," the central committee's top secret resolution of 5 March reads.

Stalin then gave the order to murder the prisoners.

They were shot in the back of the head, their bodies dumped in mass graves in Katyn and at other sites.

The names of 21,857 Polish citizens are listed among the victims on Soviet documents.

No-one is sure why Stalin ordered the killing of the prisoners rather than sending them to the Gulag.

One theory is that Stalin wanted to cripple the military of its western neighbour should Poland manage to re-emerge as a state after the war.

Despite the Soviet admission in 1990, the issue has acted like an open sore on Polish-Russian relations since then.

Sense of hurt

Moscow has refused to open its archives to Warsaw and Russian commentators still sometimes peddle the lie about German responsibility.

Until now, Russia has not commemorated the massacre, so Vladimir Putin's invitation to his Polish counterpart, Donald Tusk, to jointly mark this year's 70th anniversary is significant.

"In Polish-Russian relations, definitely, it is unprecedented," former Foreign Minister Adam Rotfeld told the BBC.

Mr Rotfeld co-chairs the Polish-Russian commission on difficult issues and he has been involved in paving the way for the diplomatic breakthrough.

"The process was initiated by Gorbachev and Yeltsin. Both of them did not have enough courage to invite the Polish president or the Polish prime minister to come to Katyn and I have to say that what Vladimir Putin has done, it is a kind of signal that something is changing," Mr Rotfeld said.

That change first appeared in a letter Mr Putin addressed to Poles ahead of his appearance at ceremonies to mark the 70th anniversary of the outbreak of World War II in Gdansk last September.

In it Mr Putin acknowledged Poles' sense of hurt over Katyn, adding the two countries should jointly remember the crime.

This year's unprecedented joint ceremony may be a step towards healing the wound caused by Katyn.