What agency or what kind of investigators are in charge of finding missing persons?

Question

There is no missing persons unit in the FBI

Answer 1

Start with local police/sheriff... From there you could hire a private investigator.

If it is a child I think the FBI steps in and so does the missing children's center

Answer 2

It depends on the circumstances of the case.  

Answer 3

Missing people who do not want to be missing would be like that TV show Without A Trace. It is a Branch of the FBI as far as I know, now if you want to find someone who wanted to get away, use a Private Investigator.  

Answer 4

Go to your local police department.  If it is an adult they usually require 24 hours minimum before they are determined as missing but will still do research to try to locate them.  If it is a case of trying to locate a family member such as a father that hasn't been in a person's life the police don't usually get involved and a person would have to hire a private investigator.