The FBI is like Tim Miller, if they are invited by the Lead Investigative Agency they will step in to help. Not limited to kidnapping, inter-state, or federal crimes. I've seen countless cases here at WS where the FBI becomes involved in cases.
Does the FBI Laboratory conduct examinations of evidence for anyone other than the FBI?
Yes. In addition to performing examinations for its own cases, the FBI Laboratory conducts scientific examinations of evidence, free of charge, for any federal, state, and/or local law enforcement organization in the United States. These services also may be made available to international law enforcement agencies under special agreement between the attorney general and the secretary of state.