A person who investigates or researches something, often with a nuance of deep inquiry or exploration. Used in academic, scientific, or detective contexts.
He is an investigator who unravels the mysteries of the universe.
The historian is known as a dedicated researcher of ancient documents.
Compound of 探究 (たんきゅう, 'inquiry, investigation') + 者 (しゃ, 'person'). The exact historical derivation is uncertain, but the components are transparent.