The legendary Reb Bentzion Shenker, may he be well, relates the story of "Niggun Ani Maamin" and how it reached America, in a discussion with Yisroel Werdyger.
This "haunting melody", is attributed to Reb Azriel David Fastag, a Modzitzer Hasid whose compositions were regularly sung in the court of the Modzitzer Rebbe, Rabbi Shaul Yedidya Elazar. He reportedly composed the tune in a cattle car while being taken to Treblinka. Fastag announced that he would give half of his share of the World to Come to whoever would bring the tune to the Modzitzer Rebbe, who had escaped Europe in 1940. Two men took him up on his offer and leaped from the moving train. One died from the fall, but the other survived and eventually brought the tune to the Rebbe's son in Israel, who sent his father the musical score. Click Here for full article.