A two-page letter written by Albert Einstein and his fellow scientist, warning President Franklin D. Roosevelt about the potential threat of Nazi Germany developing a nuclear bomb, is set to be auctioned at Christie's in September. The letter, penned by Einstein and Leo Szilard in a cabin on Long Island, raised concerns about the German government's support for nuclear research and the possibility of them creating powerful atomic bombs similar to those used by the US during World War II. The letter is estimated to fetch $4 million at the auction.