Allan Pinkerton, seen here with President Lincoln and Major General John A. McClernand, saved Lincoln's life by warning him of an assassination attempt
CREDIT: Gardner, Alexander, photographer. Antietam, Md. Allan Pinkerton, President Lincoln, and Maj. Gen John A. McClernand, 1862 (?). Prints and Photographs Division, Library of Congress. Reproduction Number LC-DIG-cwpb-03525.