This September 1862 photo provided by the Library of Congress shows Allan Pinkerton on horseback during the Battle of Antietam, near Sharpsburg, Maryland. Before the outbreak of war, he had founded the Pinkerton National Detective Agency. In 1861, he famously foiled an alleged plot to assassinate president-elect Lincoln, and later served as the head of the Union Intelligence Service -- the forerunner of the U.S. Secret Service.