I am not an expert, but I think the defendant(s) in this case will be eligible for the federal death penalty.
Here is a list of federal laws which provide for the death penalty. A few of them seem as though they might apply in this instance, though obviously it's impossible to be certain until more details are known about the case.
Wikipedia explains that "the death penalty applies if the victim expires in the perpetrator's custody, not necessarily by his hand."
Here is a list of federal death penalty case summaries.
What is particularly interesting to me is that one of the prisoners currently on federal death row is Alfonso Rodriguez, Jr., who was given a capital sentence in 2006 for murdering Dru Sjodin. She disappeared from Grand Forks, North Dakota, but her body was found in Crookston, Minnesota. Neither state has the death penalty, but because Rodriguez transported her across state lines, the federal death penalty statute applies. It would be really unusual if North Dakota ended up with a second (or third) prisoner on death row despite not having the death penalty.
Does anyone know if the federal death penalty can be applied if Sherry died in Montana and her body was subsequently transported across the border into North Dakota? I'm not clear on whether the actual murder has to take place after a kidnapping across a state line in order to be a federal crime, or if the fact that a body is found in another state is sufficient indication.
P.S. I am from North Dakota, and cases like Sherry's and Dru Sjodin's absolutely break my heart. There aren't many people in North Dakota, and the violent crime rate is so low that when awful things like this do happen, people are really shaken.