Etymology
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investigate (v.)
c. 1500, back-formation from investigation or else from Latin investigatus, past participle of investigare "to trace out, search after," figuratively "search into, investigate," from in- "in, into" (from PIE root *en "in") + vestigare "to track, trace," from vestigium "footprint, track" (see vestige). Related: Investigated; investigating.
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investigator (n.)
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 "to trace out, search after" (see investigation). Related: Investigatorial.
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investigative (adj.)

"of or pertaining to investigation, curious and deliberative in research," 1803, from Latin investigat-, past-participle stem of investigare (see investigation) + -ive. Journalism sense is from 1951.

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investigation (n.)
early 15c., from Old French investigacion (14c.), from Latin investigationem (nominative investigatio) "a searching into, a searching for," noun of action from past participle stem of investigare "to trace out, search after," figuratively "search into, investigate," from in- "in, into" (from PIE root *en "in") + vestigare "to track, trace," from vestigium "a footprint, a track" (see vestige).
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