Don't forget they work for police departments; you will probably need to have a police background too; I would not worry about which undergrad degree (though I would probably switch my major/minor and then not have to worry because forensic anthropology would probably be a BS automatically wouldn't it?) but get more information on what is required for the agencies hiring in your area might be?

How to Become a Crime Scene Investigator | Criminal Justice School Guide

I'd maybe try to get into the police force to learn police procedure first hand, and go to school on my own time so I would have the experience when I got the education.