A researcher says she has uncovered vital testimony by a U.S. officer whom the Nazis forced in 1943 to watch the exhumation of Polish officers killed on Soviet leader Josef Stalin's orders.

Krystyna Piorkowska said Wednesday she found the May 10, 1945, report by Lt. Col. John H. Van Vliet Jr. in archives near Washington. It provides evidence of Soviet responsibility for the 1940 massacre of some 22,000 Polish officers in the Katyn forest and other places in the then-Soviet Union.

Van Vliet's later, longer report is considered missing, which has fueled speculation that the U.S. silenced the truth.

Van Vliet was a POW in Germany when he was taken to Katyn to see the evidence.

Until 1990, Moscow blamed the Nazis, but then admitted Stalin ordered the massacre.