I was wondering about this with the recent back to back missing children in the area, and I was wondering why the FBI was involved and are they in EVERY case, I found this:

If a child is missing and possibly kidnapped, but no interstate transportation is known, will the FBI begin an investigation?

Yes, the FBI will initiate a kidnapping investigation involving a missing child "of tender years" even though there is no known interstate aspect. Tender years is generally defined as a child twelve years or younger. The FBI will monitor other kidnapping situations when there is no evidence of interstate travel, and it offers assistance from various entities including the FBI Laboratory.Source: FBI — Frequently Asked Questions

Now you know...