Well I have to admit, my understanding of what constitutes terrorism was not solid, so I googled it. It seems the statements don't necessarily need to be political in nature.
The FBI defines domestic terrorism[1] as:
> "Domestic terrorism" means activities with the following three characteristics:
> - Involve acts dangerous to human life that violate federal or state law;
> - Appear intended (i) to intimidate or coerce a civilian population; (ii) to influence the policy of a government by intimidation or coercion; or (iii) to affect the conduct of a government by mass destruction, assassination. or kidnapping; and
> - Occur primarily within the territorial jurisdiction of the U.S.
The first point, I imagine mass shootings qualify as acts dangerous to human life that violate law.
For the second point, I would argue that sending a manifesto[2] to media with nuggets like what follows are designed to intimidate/coerce a civilian population
"Thanks to you, I die, like Jesus Christ, to inspire generations of the Weak and Defenseless people — my Brothers, Sisters, and Chil- dren —that you fuck.
Like Moses, I spread the sea and lead my people —the Weak, the Defenseless, and the Innocent Children of all ages that you fucked and will always try to fuck —to eternal freedom. Thanks to you Sinners, you Spillers of Blood, I set the example of the century for my Children to follow."
Lastly, the attack occurred on US soil.
1 - https://www.fbi.gov/about-us/investigate/terrorism/terrorism...
2 - https://schoolshooters.info/sites/default/files/cho_manifest...