Russian Prime Minister Vladimir Putin attended a memorial ceremony Wednesday for 22,000 Polish prisoners of war killed by Soviet security forces during World War II.

Polish Prime Minister Donald Tusk accompanied Mr. Putin at the joint commemoration of the April 1940 Katyn massacre, near the modern-day Russian city of Smolensk.

Mr. Putin said for 50 years, the Soviets told what he called "cynical lies" to cover up the truth about Katyn.

Prime Minister Tusk urged Mr. Putin to ensure that all Soviet-era archives related to the massacre are open to researchers.

U.S. Secretary of State Hillary Clinton said in a statement that the meeting of the Polish and Russian leaders is a sign of hope for a bright and peaceful future.

Soviet dictator Josef Stalin ordered his secret police to execute 22,000 captured Polish army officers and other prisoners of war.  The killings took place in several locations, but Katyn has become the chief symbol of loss.

Russia blamed the killings on Nazi Germany until former Soviet President Mihkail Gorbachev admitted in 1990 that the crimes had been committed by Stalin's dreaded secret police - the forerunner of the KGB.