調查官은 郵遞夫에게 물었습니다.
Whats the difference between an investigator and a detective?
형사 is a rank in the police. A 조사관 does not work for the police but, for example, an insurance company.
Detectives usually work to solve crimes but investigators do research/spy to find people or find what people are doing in secret.
In English the words are interchangeable.