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 work was highly regarded by FBI Director J. Edgar Hoover, who once stated he is "the most knowledgeable man on crime in the U.S." — even granting Cooper access to FBI case files. Cooper is widely believed to have written several books and articles under Hoover's supervision.