noun
Typically refers to a police or law enforcement investigator working on criminal cases. Not a general term for any researcher.
捜査員が現場に到着した。
The investigators arrived at the scene.
彼はベテランの捜査員だ。
He is a veteran investigator.
調査員 is a general investigator or researcher, not limited to criminal cases.
Compound of 捜査 (sōsa, 'investigation') and 員 (in, 'member, personnel').