Another report that Van Vliet made a few days later in Washington is considered missing. This has fueled speculation that the U.S. government helped cover up Soviet responsibility for the massacre out of fear that saying the truth would anger Stalin, whom the Allies were counting on to help them defeat Germany and Japan in World War II.
For decades, Moscow had blamed the Nazis for the massacre, but then in 1990 admitted that Stalin had ordered it. The massacre further embittered Poland's relations with Moscow, which has refused to consider the massacre a genocide and has been reluctant to prosecute any living henchmen.
Van Vliet was a POW in Germany when he was taken to Katyn to see the evidence.
In the document that Piorkowska found, Van Vliet tells an interrogating U.S. officer that he saw the exhumation of some 3,500 corpses in tailor-made, little-worn Polish uniforms. They were all killed with a shot to the back of the head.
He said the "decomposition of the corpses and the nature of the undergrowth undisturbed" on the graves indicated they must have remained there "over a year possibly three or four."
"The belongings removed from the corpses all indicated death in the months of February, March or April, 1940," Van Vliet said.
The area was under Soviet control then. Germany's Nazi regime discovered the graves in early 1943 after invading parts of the Soviet Union.
The report by Van Vliet named other Allied POWs who witnessed the exhumation with him. Some of the names were not known previously to Polish historians.
Piorkowska told The Associated Press that the deposition was a signal to "go on searching, that there are still more documents to be found."
"This offers new names and suggestions of where to seek new evidence," she said, adding that she was seeking to get in touch with the other POWs' families.
Van Vliet made a new report in 1950 to help a U.S. investigation of the Katyn massacre. That document is available but is considered less valuable because of its distance in time from the actual event.