The Collaborative International Dictionary of English v.0.48:

Investigator \In*ves"ti*ga`tor\, n. [L.: cf. F. investigateur.]
   One who searches diligently into a subject.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Researcher \Re*search"er\ (-[~e]r), n.
   One who conducts research. In the field of scientific
   research, also called an investigator or scientist.
   [1913 Webster]

WordNet (r) 3.0 (2006):

investigator
    n 1: a scientist who devotes himself to doing research [syn:
         research worker, researcher, investigator]
    2: someone who investigates
    3: a police officer who investigates crimes [syn: detective,
       investigator, tec, police detective]