You don't see any openings for it because NO ONE just "becomes" a criminal investigator.
You apply at a police department, get hired, attend the academy, go through field training, and spend 3-5 years (minimum) in patrol learning the basics of law enforcement.
After all that, if a detective or investigator position comes open in your department, then you can put in for it and maybe you get selected.
Usually, a position in homicide requires a few years investigating other crimes first to teach you the basics of investigation.
There is no guarantee that you will ever make one of those spots. If you don't want to work in patrol, then Law Enforcement may not be for you. You don't want to wind up miserable for 20 years because you didn't get the investigative spot you wanted.
The only way to be assured of an investigative job right off the bat is to apply and get hired in a Federal Agency.
As a side note, a Criminal Justice Degree is not necessary to get hired into Law Enforcement. Any degree is just as good as any other. If you are really not interested in anything else, CJ is a fine major. But Chemistry, Literature, Biology, Art History, or Political Science are all just as good as far as a police department is concerned.
-Citicop.
Sometimes there's Justice...
and sometimes, there's Just Us
1*
In memory of DCLaw- EOW@RealPolice 02-20-2007.
We won't rest 'till we find the mutt.