They don’t need a warrant. They got permission from The Weiner to look at his device (no pun intended).
When it comes to Anthony Weiner, the jokes write themselves.
You can bet that according to Lynch they need a warrant. And other to obtain one she will want the FBI to agree to only look for one specific thing and to return or destroy the rest.
And you can bet that there are hundreds of entities that don’t want those emails exposed. Politicians, Media, and companies, and others. It would be a good guess that Lynch is one of them. They are going to fight tooth and nail to greatly limit this being exposed. They may even throw Hillary under bus to cover up the rest of it.