The suspect is believed to be Robert Card, a 20-year US Army veteran from Bowdoin Center, Maine.
(according to Telegrams so take with grain of salt)
EDIT: Name confirmed by news.
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According to law enforcement, CARD recently reported mental health issues to include hearing voices and threats to shoot up the National Guard Base in Saco, ME.
American society seems to put so much pressure and anger inside people that they utterly break.
Nazis on the other hand haven't been broken by a societal design that drives people to want to die and to want to take as many people with them as possible. They are ideological true believers that fully believe their acts are accelerating the move towards their fascist goals and are willing to die for it.
I think this one is starting to look like mental health rather than ideology as he's reported hearing voices, added some more info to the OP.