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:

investigating \investigating\ n.
   the work of inquiring into something thoroughly and
   systematically.

   Syn: investigation.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

investigating
    n 1: the work of inquiring into something thoroughly and
         systematically [syn: investigation, investigating]