The I Am Alone, flying a Canadian flag, was a rumrunner sunk by Coast Guard patrol boats in the Gulf of Mexico in March, 1929. The USCG knew that the ship was not a Canadian owned-and-controlled vessel but the proof went down to the bottom of the ocean when it was sunk. The Canadian Government sued the United States for $365,000 and the ensuing legal battle brought world-wide attention. Elizebeth Friedman decoded the messages transmitted by the I Am Alone, and those messages proved that the boat was, in fact, not a Canadian owned-and-controlled vessel. The case actually went to a Commission, whose final report was issued in 1935. They found, thanks in part to Elizebeth Friedman, that the owners and controllers of the vessel were not Canadian and used the boat primarily for illegal purposes.
For more information, see, for example,
“All Necessary Force”: The Coast Guard And The Sinking of the Rum Runner “I’m Alone” by Joseph Anthony Ricci, 2011, or
“Listening to the rumrunners” by David Mowry, 2001.
The above image is courtesy of the Elizebeth S. Friedman Collection at the George C. Marshall Foundation, Lexington, Virginia. (If you make use of the image, please acknowledge the Marshall Foundation.)