Group at Secret Service headquarters at Antietam, Maryland. Visual material from the records of Pinkerton's National Detective Agency. Pinkerton, Allan, 1819-1884, Secret service, Union, Maryland, Antietam, 1860-1870, United States, History, Civil War, 1861-1865, Covert operations, Union.

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Keywords: --1819-1884., ., allan, pinkerton, secret, service--union--marylan