Almost seventy years ago a Nazi propaganda film was shot inside the Warsaw Ghetto. It depicted the lives of Jews, most of whom went on to perish in a German concentration camp. Much of the film has been shrouded in mystery. The Nazis never completed it and it is not clear why they made it.One Israeli filmmaker was determined to explore the origins of the film and this week her documentary investigation is being released in American cinemas. From New York, Tom Brook reports.