I thought the federal charges were enough to do the search.
They haven’t released the cause of death.
It's enough to do a search, but not the search that it would take to actually find him. The FBI is not going to want to devote the resources needed to hunt the backwoods and country just to find someone for bank fraud charges. That's just not going to happen. They're essentially using those charges to actually find out where he is and keep tabs on him for when they actually put out the warrant for his arrest in her death.
Even if they do arrest him for those charges, he can make bail the next day and they know it. Hence, actually getting his DNA if you find anything on her body.