You cannot be charged with murder in Virginia if you are just an accessory, however often the sentencing for accessory to murder before the fact is the same as murder, 20 years TO life. So while the charge isn't the same, the penalty can be.
I am curious as to why the FBI is getting involved. I wonder if they are going to make a federal case out of this. In Virginia the two barely miss one of the capital murder qualifications, the murder of a 13 year old is a capital crime if the perpetrator is 21 or over. But that is state law. Since this involved a juvenile and they took the body over state lines, maybe that is where the FBI comes in. But sometimes when under state law you cannot get a capital crime or the death penalty, the feds step in. For example in the early 2000s when the DC/MD/Va area had the sniper killing people, Maryland wanted to try him first but they don't have the death penalty, so the feds threatened to take the case on federal charges unless Maryland agreed to let Virginia try him first as a capital crime for one of the Virginia shootings (I think the one in Fairfax), so if Virginia got him first he was found guilty of capital murder and sentenced to death. I'm just guessing here.