Synonyms for Investigation:

n.

reference, soundings, public inquiry, trawl, elicitation, opinion poll. investigate, reconnaissance, exploration. inquiry, inquisition, inquest. inspection, survey, review, examination. cognition (noun)
probe.
enquiry (noun)
exploration.
experiment (noun)
determination, trial, verification, analysis, experiment, test, benchmark, probe, proof, research.
inquiry (noun)
search, study, examination.
inspection (noun)
reconnaissance.
investigation (noun)
investigating, probe.
search (noun)
search.
thorough check (noun)
review, inquest, inquisition, probe, examination, search, inquiry, research, survey, study, inspection, analysis, exploration.

Other synonyms:

inspection, public inquiry, opinion poll, examination. inquisition, review, study, exploration, reconnaissance, inquest, elicitation. inquiry. search. reference. Other relevant words:
review, inquest, inquisition, investigating, examination, search, inquiry, reconnaissance, survey, study, inspection, analysis, exploration.

Usage examples for investigation

  1. How many of them would bear the search- light of investigation – The Lion and the Mouse A Story of an American Life by Charles Klein
  2. I have told him of your investigation into this painful case which has brought him to Norfolk. – The Shrieking Pit by Arthur J. Rees
  3. The people at the house are in sympathy with my investigation – The Millionaire Baby by Anna Katharine Green