The place to look for the origins of the literary private eye is in 19th century France with the character of Eugene Francois Vidocq.
Tags: ballistics, crime-scene investigation, criminal identification, criminal investigative unit, Dashiell Hammett, Edgar Allan Poe, Eugene Francois Vidocq, finger-printing, Kinsey Millhone, Lew Archer, Mike Ashley, Murders at the Rue Morgue, Phillip Marlowe, private detective, private eye, Raymond Chandler, Ross Macdonald, Sam Spade, Strand Magazine, Sue Grafton