Why was this a federal case?
If they caught him first,,perhaps...
Both state and feds can bring charges. May have went with whoever could bring the biggest sentence punishment.
Interstate transportation of the kidnap victim before the rape and murder. “On October 25, 2001, Nelson pled guilty in the U.S. District Court
for the Western District of Missouri to the kidnapping and unlawful interstate transportation of a child for the purpose of sexual abuse which resulted in death, and he was sentenced to death.”
https://www.bop.gov/resources/pdfs/offender_information_knelson.pdf
The girl was kidnapped from Kansas City, Kansas and her body was found in Grain Valley, Missouri, making it an interstate crime and therefore a Federal crime.