The Collaborative International Dictionary of English v.0.48:

Interrogator \In*ter"ro*ga`tor\, n. [L.: cf. F. interrogateur.]
   One who asks questions; a questioner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

interrogator
    n 1: a questioner who is excessively harsh [syn: inquisitor,
         interrogator]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "interrogator":
   asker, catechist, cross-interrogator, cross-questioner, detective,
   examiner, inquirer, inquisitionist, inquisitor, interlocutor,
   interpellator, interrogatrix, interviewer, opinion-sampler, poller,
   pollster, prober, querier, querist, questioner, questionist,
   quizzer, sampler, secret agent