Pinkertons

(LoC) "Group at Cumberland [Maryland], May, 1862. Forms part of the Records of Pinkerton's National Detective Agency."
Allen Pinkerton and his men may have been good at slipping between enemy lines, but they were terrible at estimating the size of enemy forces, oftener than not inflating Confederate troop strength to impossible numbers. General McClellan was a big fan, Abe Lincoln was a big skeptic. After the war, the agency became infamous for its anti-union mischief. - not to mention the firebombing of Jesse James' mother's house.
"In 1917, Frank Little, head of the General Executive Board of the IWW, was lynched in Butte, Montana. Author [and Marylander] Dashiell Hammett, who worked for the Pinkerton National Detective Agency at the time, was offered $5,000 to murder Little. Hammett refused, but Little was subsequently lynched by masked vigilantes, widely thought to be Pinkerton agents. The Pinkerton Agency's role in union strike-breaking eventually disillusioned Hammett and he resigned, but used his knowledge of the agency's history and exploits as material for his novels." - Wikipedia