Japanese Naval Codes - JN-39

JN-39

This was a naval code used by merchant ships (commonly known as the "maru code"), broken in May 1940. 28 May 1941, when the whale factory Nisshin Maru II visited San Francisco, U.S. Customs Service Agent George Muller and Commander R. P. McCullough of the U.S. Navy's 12th Naval District (responsible for the area) boarded her and seized her codebooks, without informing Office of Naval Intelligence (ONI). Copies were made, in a clumsy way, and the originals returned. The Japanese quickly realized JN-39 was compromised, and replaced it with JN-40.