Yours, Etc.: Origins and Uses of 8 Sign-Offs
1550s, a native agent-noun formation from investigate, or else from Latin investigator "he that searches into," agent noun from past participle stem of investigare (see investigation).
c.1500, back-formation from investigation, or else from Latin investigatus, past participle of investigare "to trace out, search after" (see investigation). Related: Investigated; investigating.