The USPS has it's own Law Enforcement Branch dealing with such actions. They are independent of the FBI, and have seniority over them in such cases.
The FBI is the lead agency in Terrorist incidents. They may, as has been already stated, be invited to join an investigation by local LEO, or the governor. There will be a team dispatched to most multiple himicides like this, just to monitor the progress of the investigation until it becomes clear the National Security isn't involved. There is also FBI involvement when a felon flees across state lines. Blacksburg isn't that far from the Tennessee line.
My money's on the locals calling them in. Even a large city would be overwhelmed with the investigation they'll have with this crime. I don't think the locals had nearly enough capacity and the Feds have most, if not all, of the goodies they'll need.
The other responses pretty much spelled out the "federal connections" quite well. Chances are that the local and campus police agencies will rely upon the FBI lab for all of the forensic work.
My guess is that the FBI is only there as an "assist" to the local police, for I haven't heard that they assumed responsibility for the entire thing.