Statistics for occurrence #1 of “Secret Service” in chapter 14 of The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 8: Soldier Life and Secret Service.:

...in any way were arrested by special civil and military agents and placed in military custody for examination. Most of this, it will be evident, had to be accomplished by means of detection known as Secret Service . As the secret-service men sit at Follen's house, near Cumberland Landing, all is ready for the advance to the Chickahominy and to Richmond. The scouts and guides are aware that there is...
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Secret Service 37 18 1 0 0 user votes

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