The Collaborative International Dictionary of English v.0.48:

Investigate \In*ves"ti*gate\, v. t. [imp. & p. p.
   Investigated; p. pr. & vb. n. Investigating.] [L.
   investigatus, p. p. of investigare to investigate; pref. in-
   in + vestigare to track, trace. See Vestige.]
   To follow up step by step by patient inquiry or observation;
   to trace or track mentally; to search into; to inquire and
   examine into with care and accuracy; to find out by careful
   inquisition; as, to investigate the causes of natural
   phenomena.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Investigate \In*ves"ti*gate\, v. i.
   To pursue a course of investigation and study; to make
   investigation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

investigate
    v 1: investigate scientifically; "Let's investigate the syntax
         of Chinese" [syn: investigate, look into]
    2: conduct an inquiry or investigation of; "The district
       attorney's office investigated reports of possible
       irregularities"; "inquire into the disappearance of the rich
       old lady" [syn: investigate, inquire, enquire]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "investigate":
   air, analyze, canvass, comment upon, consider, controvert,
   deal with, debate, deliberate, deliberate upon, delve into,
   dig into, discourse about, discuss, examine, exchange views,
   explore, fathom, go, go into, handle, indagate, inquire into,
   knock around, look into, pass under review, peer into, plumb,
   poke into, probe, pry, pry into, rap, reason, reason about,
   reason the point, research, review, scrutinize, search into, sift,
   sound, study, take up, talk, talk about, talk of, talk over,
   thresh out, treat, ventilate, winnow