Federal law takes primacy over state law; likewise, in a criminal event such as this mass shooting, both federal and state crimes are alleged. When both state charges and federal charges stem from the same event, federal jurisdiction gets first dibs unless the feds, at their discretion, allow the state to prosecute first.
Yes, I understand how it is, the question is, should it be that way? That is not how the power should run. The States should have first shot, then lastly, the feds. I believe that the Feds are overstepping their bounds. Having the Director of the FBI leading this himself, to me is proof of that.