At the start of the Civil War, Alexander quit the United States Navy and joined a band of Confederate pirates who seized Yankee ships in the Chesapeake Bay and sailed them to Richmond. Captured in July 1861, he was imprisoned in Baltimore’s Fort McHenry, but he escaped, leaping from a rampart and swimming to shore. He made his way back to Richmond and joined the city’s military police. A year later, he became commandant of Castle Thunder
http://opinionator.blogs.nytimes.com/2013/06/03/the-pirate-warden-of-richmond/?_r=0

See also:

George W. Alexander and Castle Thunder: A Confederate Prison and Its Commandant
Frances Harding Casstevens
McFarland, Jan 1, 2004 - Biography & Autobiography - 271 pages

This biography traces his entire life from his career in the U.S. Navy and the voyage with Commodore Perry to Japan, to his hiding in Canada after Lees surrender, to his editorship of Washington DCs "Sunday Gazette" and death in 1895.
https://books.google.com/books/about/George_W_Alexander_and_Castle_Thunder.html?id=NupsSGl-B2gC