I also thought kidnapping was a federal crime. The following gives some information as to what changes a state crime to a federal one when it comes to kidnapping:
Beyond interstate travel and specific victim or location criteria, other circumstances can make a kidnapping a federal offense. Kidnapping to collect ransom for a federal crime, or as part of a broader federal racketeering enterprise, can lead to federal charges. The use of interstate communication tools, such as phones or the internet, to facilitate the crime can also establish federal jurisdiction. These scenarios often involve a connection to other federal criminal activities, which allows federal authorities to intervene. For instance, if a kidnapping is linked to terrorism, it would be prosecuted at the federal level. The involvement of ransom money, including its receipt, possession, or disposal, can also lead to separate federal charges.
From http://legalclarity.org/when-does-a-kidnapping-become-a-federal-crime/
Thanks for the info!