From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]