Both feds and state could both have jxn here. What I suspect defense will do is file a motion to dismiss based on the lack of jurisdiction because there's no involvement of interstate commerce. They may wait to do it however, because right now there's a rebuttable presumption that kidnapping is out of state if more than 24 hours elapsed. So they may have a hard time rebutting that presumption and say client is innocent plus they might want to argue that some random person kidnapped her and took her out of state or whatever. Many lawyers actually prefer being in federal court too.
But once facts are out and if her body is found in state and the allegations entirely consist of activity in state, they could file a motion to dismiss based on the lack of interstate contacts. I think this would be a strong argument, especially if the only reason this case is being treated differently is because of federal death penalty. The state would then file their own complaint and he would be tried there. If activity is wholly intrastate, the feds may just agree not to prosecute at this time because they will want to preserve trying him at a later date if he should get acquitted in state court. Why would the feds want to get involved in a lengthy constitutional challenge on jxn when he can be put away by state court without those issues? In most cases they wouldn't - they would just give it back to the state. The main reason would be to give him the death penalty which he would not be able to get in state court. Holding the death penalty over him is the best leverage they have at this point to find out where the body is so why not use it? Plus, the optics of this case may be somewhat unique in that China may be pushing for death so they have incentive to keep it in federal court for now.
There may also be other reason that defense could say that the case should go back to state court, but I don't know what those would be. Most people want to remove from state to federal court in civil cases, not the other way around. Moreover, since both state and federal can charge it's not like the defense can say "hey feds, you can't charge him." They can charge him with federal kidnapping, defense could only argue 1) federal kidnapping does not apply here because no interstate conduct nor does the crime fit within any of the other fact scenarios under the federal kidnapping act; or 2) for convenience, etc. try him in state court first, then federal (that is, state a preference for trying him in state court first). State is being smart here because right now his only option is number 1 since the state has not charged him yet. Moreover, the feds would be very reluctant to treat this case differently than other cases- the whole purpose of federal involvement is national uniformity - if the state charges, their normal approach would be to defer, so if they didn't it makes it look like they are not following their own policy. They must be coordinating with the state because they get around this dilemma by the state withholding charges (though at this point, I don't know what the state could charge him with - I assume there is an IL kidnapping law, but that's probably all they can charge him with at this point).
Thanks for that good info. I have a question. If this does go to trial, and no body is found, and no evidence emerges to rule in or out BC's crossing state lines, i.e., the 24-hour presumption cannot be rebutted, would the case stay with the feds?