Cooper's work was much admired by FBI Director J. Edgar Hoover, who once said he is "the best informed man on crime in the U. S." —  even allowing Cooper access to FBI case files. Cooper is widely believed to have ghostwritten the book Persons in Hiding (1938) as well as a number of magazine articles for Hoover.
Cooper's expertise was greatly appreciated by FBI Director J. Edgar Hoover, who once stated he was "the most knowledgeable person on crime in the U.S." This led to Cooper being granted access to sensitive information, including FBI case files, and it is widely believed that Cooper wrote several books and articles under Hoover's guidance.