The trial of the Rosenbergs and Sobell on federal espionage charges began on March 6, 1951, in the US District Court for the Southern District of New York. Judge Irving Kaufman presided over the trial, with Assistant U.S. Attorney Irving Saypol leading the prosecution and criminal defense lawyer Emmanuel Bloch representing the Rosenbergs. The prosecution's primary witness, David Greenglass, said that he turned over to Julius Rosenberg a sketch of the cross-section of an implosion-type atom bomb. This was the "Fat Man" bomb dropped on Nagasaki, Japan, as opposed to a bomb with the "gun method" triggering device used in the "Little Boy" bomb dropped on Hiroshima. He also testified that his sister Ethel Rosenberg typed notes containing US nuclear secrets in the Rosenberg apartment in September 1945.