It means that this case is so big that not only will the state prosecute, but there are federal prosecutors waiting in the wings to file charges.
In general, the federal government cannot prosecute somebody for the same crime that the person faced in a state court. This is why I parenthetically asked, on what charge would this person face in federal court. If he is charged with, let’s say, aggravated vehicular homicide in Virginia, this crime might not even exist under federal law (except in D.C., on a military base, etc.). Even if this crime existed at the federal level, a person could not face a second trial (in federal court) for that particular crime. In the rare instances that the Federal government tries a person for something like a crime a person faced in state court, the federal government makes a new charge. I gave the example of denying a person their civil liberty. See the below reference. But, hey, you’re the constitutional scholar. You figure out what the federal charge would be.
https://www.wklaw.com/double-jeopardy-federal/