Earlier quoted context omitted.
Probably to understand motive, and check for potential involvement of others. And whilst they have a metric shit-ton of spying capabilities on US civilians, those are not really intended to avoid crime. (Maybe the shift of perception of what is terrorism might change that, though)
He posted a manifesto online. Think his motive was pretty clearly outlined in it.
https://en.m.wikipedia.org/wiki/List_of_mass_shootings_in_th...