https://metro.co.uk/2018/10/26/suspected-maga-bomber-idd-as-native-american-trump-supporter-cesar-sayoc-56-8079040/"
From your link:
"Online records show Sayoc, who has a criminal record, has owned companies called Native American Catering & Vending, as well as Proud Native America One Low Price Drycleaning."
So maybe it was Elizameth Warren's cultural theft that set him off...(That was not a typo)