Is the argument that, without access to his phone, they can’t stop this guy from killing more people?

Is it that they need the phone to tell them that white supremacy and assault rifles are things that exist in the US?

Do they need the phone data for a conviction?

I don’t understand what they expect to do with whatever they find that they can’t get from other sources.

They can get location history and a pretty good idea what and who he communicated with from ISP and cell logs, etc.