Exactly - Georgia has the GBI for investigation, questioning and sample testing purposes. The FBI has specific jurisdiction. Was it definitely the FBI? The GBI usually handles missing person reports in Georgia
from Wiki: "The Georgia Bureau of Investigation (GBI) is the U.S. state of Georgia's state bureau of investigation. It is an independent, statewide agency that provides assistance to Georgia's criminal justice system in the areas of criminal investigations, forensic laboratory services and computerized criminal justice information. Its headquarters is located in unincorporated DeKalb County, near Decatur and in Greater Atlanta.[SUP][4] [/SUP]"
https://en.wikipedia.org/wiki/Georgia_Bureau_of_Investigation
So why wasn't it the GBI's job to step in instead of the FBI?
Both Subie (our VI), and the "official" help find Jenna FB page have mentioned FBI involvement. I have no reason to disbelieve them.
One reason (amongst many others) for which the FBI can and does get involved is sex trafficking. Another is when they have an ongoing investigation which - for whatever reason - cross-references with a local LE case.
Just a couple of examples, & JMO.