On Monday, the U.S. National Archives released 1,000 declassified documents pertaining to the 1940 massacre of 22,000 Poles by the Soviet Union. The Cliffs Notes version? America's coverup of the infamous Katyn Massacre was more extensive than previously thought.

For years, Poles and Polish-Americans have alleged that the U.S. government suppressed information about the Soviet Union's guilt in the World War II-era murders, which were aimed at killing off Poland's military and intellectual elite. As recently as 1992, the State Department said it "lacked irrefutable evidence" in the early 1950s to substantiate claims that the USSR, not Nazi Germany, carried out the crimes. But today's documents show the concrete proof U.S. officials had in their hands in the 1940s regarding the Soviet Union's guilt. 

You can see all of the newly-released documents and maps at the National Archives site here. The AP's Vanessa Gera and Randy Herschaft got an early peek at the documents and have zeroed in on the the most surprising revelation: Secret codes sent by two American POWs to the U.S. in 1943 acknowledging evidence of rotting corpses in a state of advanced decay. In essence, "proof that the killers could not have been the Nazis who had only recently occupied the area." The POWs were Capt. Donald Stewart and Lt. Col. John H. Van Vliet Jr, per the AP:

The newly declassified documents show that both sent secret encoded messages while still in captivity to army intelligence with their opinion of Soviet culpability. It's an important revelation because it shows the Roosevelt administration was getting information early on from credible U.S. sources of Soviet guilt — yet still ignored it for the sake of the alliance with Stalin ...

A statement from Stewart dated 1950 confirms he received and sent coded messages to Washington during the war, including one on Katyn: "Content of my report was aprx (approximately): German claims regarding Katyn substantially correct in opinion of Van Vliet and myself."

However displeasing, the decision to suppress the evidence during the war had its merits: the U.S. needed the Soviet Union's help in fending off Germany and Japan. However, why the secret codes were concealed until now is less defensible. Documents show that Stewart was ordered in 1950 " never to speak about a secret message on Katyn," a suppression tactic historians are now calling "potentially explosive."  

Elsewhere in the trove of declassified documents are maps of the mass graves. 

The above map shows the location of the Polish officers' graves, including a "detailed plan of the 8 mass grave locations." Below, is a map drawn by German Army reporter Gregor Slowenczik who documented the Katyn excavations for the U.S. Counter Intelligence Corps in 1946. 

According to the National Archives, after Slowenczik's death, his wife "provided this map, along with photographs and other testimony remaining from her husband's visit to Katyn, to the United States Forces." You can see more of the declassified materials here. Below is a documentary about the Katyn Masscare held by the National Archives: