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Let me provide you a short excerpt about what were American governments thoughts on atrocities commited by it's Soviet "ally":

Karski met with Polish politicians in exile including the Prime Minister, as well as members of political parties such as the Socialist Party, National Party, Labor Party, People's Party, Jewish Bund and Poalei Zion. He also spoke to the British Foreign Secretary Anthony Eden, giving a detailed statement on what he had seen in Warsaw and Bełżec. In 1943 in London he met the journalist Arthur Koestler (author of Darkness at Noon). He then traveled to the United States and reported to President Franklin D. Roosevelt. In July 1943 Karski again personally reported to Roosevelt about the situation in Poland. Karski met with many other government and civic leaders in the United States, including Supreme Court Justice Felix Frankfurter, Cordell Hull, William Joseph Donovan, and Stephen Wise. Frankfurter, skeptical of Karski's report, said later "I did not say that he was lying, I said that I could not believe him. There is a difference."[7] Karski presented his report to media, bishops of various denominations (including Cardinal Samuel Stritch), members of the Hollywood film industry and artists, but without much result. His warning about the Yalta solution and the plight of stateless peoples became an inspiration for the formation of the Office of High Commissioner for Refugees after the war.[8] In 1944 Karski published Courier from Poland: The Story of a Secret State (with a selection featured in Collier's six weeks before the book's publication),[9][10] in which he related his experiences in wartime Poland. The book was a major success (a film of it was planned but never realized) with more than 400,000 copies sold alone in the United States up to the end of World War II.[citation needed]

Please have a look at the bio of this brave and commited person:
Jan Karski