A person who conducts an official inquiry, inspection, or examination. Often used in contexts like insurance, accidents, or government surveys.
The insurance company's investigator examined the accident scene.
He works as an examiner for the tax office.
調査員 is a general investigator carrying out fieldwork, whereas 主査 is the lead investigator who oversees and directs the investigation.
Compound of 調査 (investigation) + 員 (member, person). A straightforward Sino-Japanese formation.